My layers free range and have done damage to onions, garlic and chives at different stages. Not crazy about the tast of eggs after those times or after broccoli and brussels sprouts rampages. Those areas are off limits now until moulting time. Maybe experiment slowly adding it. Garlic has so many great health and preventive benfits that here must be a thread giving dosages.
Garlic is one of those things you aren't supposed to feed to chickens much. It has oxalic acid in it which all birds have a hard time processing in their kidneys (this according to Dr. Ross Perry, one of the best bird vets in the world). So just be careful not to feed them very much of it. I've read not to give any to them at all but really, I don't think a little bit hurts them, there's too many people that have had the chickens eat their garlic plants without keeling over.