Garen is honestly not best built as a dps under any circumstances. Build him tanky, with warmogs, merc treads/ninja tabi, FoN, sunfire cape, thornmail, atma's. Standard tank build stuff. If your team is squishy, drop something for an aegis.
You should be building garen with the idea of an immovable wall that can never be forced out of lane for very long. He can't be a reliable ad carry because of his lack of stuns, slows, snares, and dashes. I know, I've tried ..
He has a minor move speed steroid, and a decent cc clearer, but he is a very powerful off tank, and almost never banned. Just watch out for ignites!
Hope it helps!