I have a pre-HK-lawsuit ATI GSG5 which has been a ton of fun. The GSG522 is basically the newer version that has all the cosmetic changes needed to make HK happy. (Umarex paid HK for use of their trades so that they can sell the Walther-made "HK".)
The ATI GSG5's have been around forever now, and all bugs are known and with known fixes, plus a huge aftermarket of parts. And - ATI has excellent cust service. Umarex has better cosmetics, but the jury is out on all else considering the soiled rep of their "Colts".
Been yipped at on another thread here about links to other boards, but whatever - we help each other. Here is the link to the dedicated GSG subforum over at ARF where "Chris" from ATI hangs out to help. (He is at other forums too, like the 1911 forums to help out with GSG 1911/22 questions/issues. )
Go here for EVERYTHING .22, including a subforum dedicated to tactical 10/22s, like the Marauder G36 kit and Troy T22 kit.
And here for anything tactical .22 including stuff covered at the above forums..
22tactical.com 22 Tactical Firearms GSG5, GSG5P, GSG5PK, Sig 522LR AR15
Lastly as has been mentioned; the SIG USA-built, SIG USA-branded 522 copy of their 556 rifle is the absolute best .22 evil black rifle on the planet, period, end of argument, no question, done. Just add an optic and your GTG.
My SIG 522 eats any brand of .22LR ammo from T-bolts to Aguila 60gr SS Snipers. The BlackDog-made-for-SIG polymer mags work great and give you the full 28rds without a hitch.
You could take a SIG 522 into war it is THAT good of a rifle.