Nice guide, really helped me out :)

1st of all, I would suggest trading Guardian Angel for Warmog's Armor, it will boost the damage from Atma's Impaler and give more hp.
Secondly, this is a tanky dps Garen build, unlike a Carry Garen that generally get's Infinity Edge, Phantom Dancerx2, Boots of Swiftness and Bloodthirsterx2.
Consider switching Youmuu's Ghostblade to Last Whisper, more ad and armor penetration, as compared to the small crit bonus to Judgment.
Finally I would suggest getting mastery points into Havok for the epic damage boost.
Executioner , btw, is an epic choice on Garen due to giant bonus when using Demacian Justice.

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Edit, if I helped, +rep.
If you wanna talk about it feel free to PM, I actually check them a few times a day.