The first time I heard this name, it was in the form of Garreth (with two Rs) and pronounced GAHR-ith. I like that pronunciation better. But GAR-ith is also handsome. It's a strong name, and yet it derives from a word meaning 'gentle'. I love the irony.
There is something whimsical and medieval about this name. Highly uncommon but not disregarded. It would be nice to see this name as a common name because it's charming in my opinion.
I'm not crazy about names with the ''th'' ending, but this isn't such a bad name, as long as the guy isn't called Gary. I hate the name Gary, as it makes me think of annoying middle-aged men who find themselves hilarious and very intelligent and strongly believe that anyone who disagrees with them and fails to laugh at their jokes has issues, is a liar and a fake, or just happens to be dumb. So, if the guy is called Gareth, not Gary, go for it.
My brother's name is Gareth, and because I live in Wales I know quite a few people with the name. It's popular around here it seems but I'm not sure about elsewhere. I like the name, it's simple but classy, and shows a pride in being Welsh.