Update:
With the rework of Gragas on patch 4.5, AD Offtank Gragas has lost everything he had: the AD steroid, the damage reduction, the AD scaling and the attack speed slow.

So this build is now obsolete.

To view Gragas's abilities before the rework, you can have a look here on League of Legends' old Wikia page of Gragas

Gragas is objectively one of the best top laners there is in League of Legends. Let me explain why:

He has only one spell that scales with AD, his Body Slam, but he has one of the best AD steroid in League of Legends, Drunken Rage, giving up to +70 AD. Your auto-attacks will hit like a truck, and combined with the slow of Body Slam once you get in range with your opponent he's bound to lose at least half of his max health.

What is more, you have an excellent sustain because every time you cast a spell, Happy Hour will regenerate some health, and neither the enemy top laner, the jungler or the minions will be able to scare you because they'll do peanut damage thanks to the damage reduction of Drunken Rage, reducing up to 18% of the damage received.

Pros

+ Second best flat AD steroid in the game with Drunken Rage
+ Insane sustainability during laning phase with Happy Hour and the damage reduction of Drunken Rage
+ Nice AD scaling dash with low cooldown and slow to chase enemies
+ Can easily trade with top lane AD champions thanks to the attack speed slow from Barrel Roll
+ True Top lane boss, AD Offtank Gragas counters most of the top lane champions

Cons

- Can be kited by champions with hard CC or long range
- Utility falls off late game

Runes

Even though you have two spells that scale with AP, most of your damage will come from your auto-attacks enhanced by Drunken Rage, so you'll need Greater Mark of Armor Penetration. For the rest, typical runes for a Top lane offtank champion.

Masteries

Happy Hour

This passive was useless for AP Gragas, but it is awesome with AD Offtank Gragas. The more health you have, the more you will regenerate each time you cast a spell.

Barrel Roll

We won't be using this to deal damage, but for its attack speed slow, which is great if you have to trade against auto-attacker Champions like Tryndamere.

Drunken Rage

This is the ability that makes AD Offtank Gragas the true Top lane boss. It transforms Gragas into a drunken monster :

- the second best AD steroid in the game with a bonus up to +70 AD for 20 seconds, every 25 seconds (nearly permanent!)

- a damage reduction (both physical and magic damage) that last as long, that makes Gragas unkillable, unpokable, ungankable, ...

- infinite mana regeneration

Be careful this is a channeling ability, you have to wait the channel to be completed to receive the full buffs.

Use Drunken Rage somewhere safe where you won't be interrupted, before the beginning of teamfights and 1v1.

Body Slam

A great gap closing spell that scales from 30% to 70% of your AD depending on how much you upgraded it. Don't be afraid to play as aggressive as possible during laning phase, the combo Body Slam + auto-attack enhanced by Drunken Rage and Sheen will obliterate the enemy.

Explosive Cask

Even without any AP this uti deals some nice damage.

However as AD Offtank Gragas we will use to for its crowd control ability:

- to pull an enemy back to you if you are chasing him

- to push back the enemy team if you want to escape or save an ally

- to seperate the enemy ADC or AP carry from the rest of his team

Level up Body Slam once first to be able to escape if there's an early gank.

Max Drunken Rage to benefit from maximum AD steroid, damage reduction and mana regeneration.

Max Body Slam as your second spell to increase the AD scaling, from 30% to 70% of your AD.

If you're facing a heavy auto-attacker champion like Tryndamere you can max Barrel Roll second to nullify his damage with its attack speed slow. Otherwise, max this spell last as you won't be doing any damage with it and will mainly use it for the attack speed slow.

As usual, level up your ulti Explosive Cask whenever possible.

How to build AD Offtank Gragas

During laning phase, your kit allows you to do an insane amount of damage. So you should take advantage of that by buying items that boost your damage, but most importantly attack speed to capitalize on Drunken Rage AD steroid (the more attack speed you have, the more often you'll land auto-attacks that have been boosted with +70 AD).

However, after the laning phase is finished, I noticed that AD Offtank Gragas's damage falls off quite a lot, because he won't be tanky enough to be in melee range during teamfights, he won't be doing as much damage as before, and he can be kited by long range ADC and mages. So starting from mid game, once you have two offensive items, you should start building more tanky and gradually change your role from Offtank to Tank.

Offensive items

Trinity Force

The Trinity Force is really the core your build, it gives you everything you need.

The Sheen part transforms your Body Slam + auto-attack combo into a huge burst
The Zeal take full advantage of the AD steroid.
The Phage allows you to stick to your enemy along with the Body Slam dash.

Blade of the Ruined King

Similarly to Shyvana, you are a bruiser that benefits from attack speed, so Blade of the Ruined King is a perfect item for you. Added to the +70 AD, your auto-attacks will also deal part of your enemy's current health.

Zephyr

When i take the Ninja Tabi I like to buy Zephyr to have the Tenacity I need, plus some nice attack speed bonus.

Wit's End

Another attack speed item, it is good if your team deals lots of magic damage, or if you're facing an AP champion like Teemo or Vladimir.

Ravenous Hydra

This offensive item doesn't give you any attack speed, but it can help you farm better. The combo Body Slam + Ravenous Hydra can clear minion waves in no time.

Choose either Blade of the Ruined King or Ravenous Hydra, one lifesteal item is enough.

Defensive items

Frozen Heart

Buy Frozen Heart if you're facing a heavy AD enemy team. The attack speed slow of Frozen Heart syngergizes well with the one of Barrel Roll.

Randuin's Omen

Another nice tanky item with a good active for teamfights.

Sunfire Cape

This can help you clear waves while staying tanky.

Warmog's Armor

Warmog's Armor gives you a massive amount of health. The more health you have, the more you will regenerate with Happy Hour.

Spirit Visage

Its unique passive increase your health regeneration from Happy Hour by 20%.

Boots

Ninja Tabi

I like this boots because it makes Gragas dominate top lane even more. If you get this, buy Zephyr along with it.

Mercury's Treads

Another possibility of upgrading your boots, especially good if the enemy top lane is AP or has good crowd control abilities like Cho'Gath.

Start with a Doran's Shield, a Health Potion and a Stealth Ward.

Rush the Sheen, this will five you an insane burst. You can then finish the Trinity Force as soon as possible, or get the Boots or Ninja Tabi before.

Choose two offensive items with at least one attack speed item and one lifesteal item: Blade of the Ruined King, Zephyr, Ravenous Hydra or Wit's End.

When laning phase is finished, start building tanky. Get items like Frozen Heart and Randuin's Omen if the enemy team is mostly AD, or Warmog's Armor and Spirit Visage if they are mostly AP.

At last, upgrade your boots, trinket and elixir.

Here are some videos of Youtubers playing AD Offtank Gragas. These were posted before my guide was published, so they don't follow my build : they are purely here to illustrate the play style of AD Offtank Gragas.

AD Offtank Gragas is a super fun and effective way to play Gragas in the Top lane.

This build has been fully tried and tested for normal games at least.

However, I don't play Ranked games so I don't know if it works as well as normal games. If you tried it on Ranked games, let me know what you think !

If you have success with this build, you can also send me screenshots so that I can include them in the guide.

I'd be delighted if you can give me a feedback (either positive or constructive negative feedbacks) on the presentation of this guide or the build itself !