Never heard of gamersgate, are they good? trusted?
Gamersgate
Getgamesgo
Goodoldgames
Direct2drive
Greenmangaming
Along with Steam, Origin and rainforest are the main digital distributers I browse for deals, and I consider them pretty much safe
Though bear in mind
unless they actually specify "steam key" or whatevers relevent, you should not count on your purchases working on steam (they all offer downloads though, which imo is nicer as you get to back up the installer)
I did have a stroke of luck a few months back, when I purchased Civ 5 during a sale for £6, I found that it actually activated on steam, which was great, so you do sometimes get lucky like that
Occasionly once in a blue moon you can find a good deal on "flea" bay too.
For example I actually bought a retail sims 3 + pets combo pack sealed for £12 a few weeks back as an xmas gift to someone (don't ask lol) which is a bloody good deal as most retailers want £45-50 just for the digital version
Its the countless 100's of "steam/origin key scan" vendors you need to be weary of. I'm not going to list an example of the bad ones because I don't feel people should be exposed to that so close to xmas
I will say keys4me stands apart from them, and seems to be pretty reliable with a fairly large community following on the steam network itself
Hope this was helpful
p.s. in response to your original question I enjoyed several years of the original UT before it died out through lack of support. In truth its probably the only fps game I had my heart in

I feel the successive titles didn't live up to par, not even 2k4.
I would love nothing more than a replica remake or ut99 with a new engine, infact I'd hop skip and jump to fork out 40 quid for it tomorrow
Though I'd imagine since I'm older my reflexes have gone to pot and I'd suck
Happy Holidays