Hi there, you can try mysore garage on street 48 gate 18

They have a lot of american car experience. I've seen them working on a few hummers before.

I've been taking my caddy to them since they've opened on advise from a rent-a-car buddy of mine and have had no problems with them so far. Reasonable prices as well.

Here's their website, check them out.

www.mysoregarageqatar.com

or call this guy henry, he'll hook you up.
+974 55 85 6229

Hope this helps.