Why Eclipse?
It's a decent early item because it builds from Serrated Dirk and stays strong late game with arpen and omnivamp buffs. The lethality+arpen combination means you don't have to discriminate team fight targets but can just kill whatever gets in your way, which due to mobility issues is how garen needs to be played.

I take it top/mid or jungle.
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Bans:
If playing top-lane ban Vayne. If playing mid or jungle ban Alistar. Even though you won't be laning against Alistar, he will make you want to uninstall. No other champion counters garen as effortlessy.
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Spells:
take Ghost/ Smite if playing jungle. take Ghost/ Teleport otherwise.
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Runes:
take Ravenous Hunter against a melee team and Relentless Hunter against a ranged team (also helpful against a ranged lane opponent to dodge skill shots). take Second Wind and Overgrowth or Unflinching if they have 3+ sources of stuns. take Conditioning if your lane opponent is something you can't trade with like Singed. take Conditioning/ Revitalize if playing jungle.
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Items: Build in the order listed unless otherwise noted.

Core Items:

Eclipse: most well-rounded item since the nerf of Goredrinker. Gives you the lethaltiy and arpen boost needed to snowball right after building up your damage output with Wit's End/ Blade of the Ruined King + Chempunk Chainsword

Blade of the Ruined King / Wit's End gives you good wave clear and fighting ability.

Chempunk Chainsword You need anti-heal, you need health, you need damage. This provides all 3 for a cheap price.

Sterak's Gage always get this, but if they have 4 AP you can swap it for Maw of Malmortius

Spirit Visage you rarely get around to it but as a 6th item it will synergize with your omnivamp, W, Sterak's Gage and Blade of the Ruined King. If you went the Wit's End route then build Randuin's Omen instead.

Boots:
Berserker's Greaves take this if both your lane opponent and enemy jungler are tanks. You can sell it later for Plated Steelcaps

Plated Steelcaps: Getting these pretty much every game, either sooner or later. If you're laning against a burst mage and need the steelcaps due to their team composition, then delay purchasing it until after Wit's End and just run around with Boots for a while.

Boots of Swiftness if their entire team is ranged and less than 3 of them are auto-attackers


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GamePlay: (league replay files)

carrying toplane vs mordekaiser [strategy here was to proc predator to bait his shield, back off, then re-engage with ghost to all-in]
https://drive.google.com/file/d/17OYROK8yEkVpdn1HQ-FTpIb9I8jcdInN/view?usp=sharing

carrying as jungle garen
https://drive.google.com/file/d/1lObkUdoYkWE-pcE8aHVEO1U_FUXO8x4S/view?usp=sharing

carrying jungle [started with the blue camp]
https://drive.google.com/file/d/1TUZHiZKPUcE6xIpLPZjKiM6Q8uDwyqCj/view?usp=sharing

carrying top vs heimerdinger (very tough matchup. took the wits end first item. all you can do against heimer is cede ground until you get a gank.
https://drive.google.com/file/d/1hJQu-chMyMQ-vhUhgl5BFmBA8rz8DCDr/view?usp=sharing

carrying mid vs ryze. built Wit's End first, then Plated Steelcaps then Eclipse.
https://drive.google.com/file/d/155H4ED13fjYgxUdxstqo_0embFuZzM8N/view?usp=sharing

carrying mid vs orianna + entirely ranged team. i took Boots of Swiftness for this. I also took Unflinching because they all had CC.
https://drive.google.com/file/d/1w9kSd8YC0EiFmIYNUfKoyl4i2sAtUD0M/view?usp=sharing

carrying mid vs katarina
https://drive.google.com/file/d/1vG3BCLtBmRflg7_iWAadut--oYa7xDJZ/view?usp=sharing

carrying top vs illaoi (took ravenous hunter & hydra for this. after finishing spirit visage with 23% omnivamp, their jinx couldn't even dent my health bar in the final fight)
https://drive.google.com/file/d/1tdaT-M5UowtKtb09QeDQhsb1b1wtmTIb/view?usp=sharing

(If you don't know how to watch a replay (.rofl) file there are instructions on google.)