Hi,
A little introduction, this is my GP crit guide.
Also this is my first guide so feedback is appreciated!

Note: This is the way I play GP, I'm not saying it's the best but it works for me :)

For runes I run with:
Marks:
Greater Mark of Desolation x9 For a chunk of ArP
Seals:
Greater Seal of Critical Chance x9 To increase my base crit and get closer to 100% endgame.
Glyphs:
Greater Glyph of Critical Damage x9 For the extra damage on my crits ofcourse!
Quints:
Greater Quintessence of Critical Damage x2
Greater Quintessence of Critical Chance x1
These quints are my preferance, you can either go full crit damage, crit or ArP
Why I don't take cdr or AP or anything, I think that you can get enough cdr from your boots and masteries.
Ap... well I don't think it benefits you as much as AD/Crit/Critdamage does.

Classic 21-0-9,
I tend to swap my Awareness for Expanded Mind in 5's to stay in lane longer, ofcourse packed with Clarity instead of Ignite.

Starting with the famous Brawler's Gloves which I turn into a Avarice Blade, after that it is my main prio to rush IE as fast as I can.
Building IE I start with Cloak of Agility to get my crit up and farm till I have enough for a B.F Sword. Once we have those I tend to complete my IE in one time, I think the extra money needed is too little after your Pickaxe so I stay laned a bit longer.
After your IE check your opponents for their mobility, if they have upgraded boots I take my normal's followed by Ionian Boots of Lucidity. If they still have normal boots I mostly pick a Zeal for the extra damage and crit since you can outrun them with our lovely raise morale.
Turn your Zeal into a Phantom Dancer to boost your crit, movementspeed, the AS is a nice little extra imo.
After that I mostly adapt to the enemy team, Banshee's Veil or Atama's Impaler you know what to do :)
Depending on my farm / kills I pick a Trinity Force or Frozen Mallet, with these slows you have even more potential to kill.
Pick a Phage more early if the enemy likes to gank ofcourse!
Feel like dominating? Bloodthirster gives you more damage so bigger crits! (and lifesteal ofcourse..)
Added a second page based on some 5v5 including defensive items which are needed obviously

With this build we want our Parrrley to do the damage so pick this when you can!
Ofcourse pick your ultimate when possible and take scurvey or raise morale which ever is needed more.
Hard cc / a lot of damage, go for scurvey.
If you need more speed to chase or flee take morale, sounds quite easy doesn't it?

I usually run Flash + Ignite
For escaping Flash always saves your *** if used right, problems mostly occur when you just miss a little range and therefor bash against the wall resulting in a death.
I also find it usefull against fleeing enemies but this isn't new for you guys so I won't go on about this :)

Ignite reduces healing and deals damage + damage over time, it may not be a lot but can be just that extra burst you need! also nothing new :)

Pros
- Great damage late game
- Global ultimate
- aoe speedbuff
- build-in cleanse
- Is a pirate! :D

Cons
- Squishy, especially early game and if you decide not to take any defensive items
- Item dependant