It'd be worth getting a scan tool. A decent one. A lot of people here use the Torque app, they purchase the little bluetooth dongle thing and put Torque on their Android phone, but of course this requires having a smartphone (I don't have one lol).
O2 sensor socket - worth having on hand just in case, can be used on a lot more than just your GMT, and is only like $8-10.
35mm deep-well socket - this is used to get the axle nut off on the front, if you wanna get into fixing your own disconnect, replacing hub assemblies and CV joints, etc.
Torque wrench. This is fairly important on a bunch of parts on this platform. It's trivial to find the proper torque spec on this site or in a Haynes manual, but you need the wrench to be able to do it. Get a 1/2" one for the heavy-hitter, 100+ lb settings, and a 1/4" for the inch-pounders. A 3/8" may not be necessary if the 1/4" and 1/2" ranges overlap. Get a 1/2" to 3/8" adapter at Wal-Mart to use your 3/8" drive tools, I got one that comes packaged with a 1/2" universal joint also for like $6. Torque wrenches can be a little pricey, if you wanna cheap out and go the Harbor Freight route it's a viable option.
6" C-Clamp, OR Piston Retractor tool. You can find the piston retractor tool in the brakes aisle at Harbor Freight. Looks like a piece of metal, either rectangular or curved kinda like a brake pad, with a threaded rod running through it. A 6" C-Clamp can be used as a substitute. This is to back the brake pistons into their sleeves when doing your brakes.
Retrieval kit. Dirt cheap at Wal-Mart where I got mine, came with a telescoping magnet, telescoping inspection mirror (can be used to inspect hard-to-reach parts of the vehicle, not just for tool retrieval) and a little clamp deal. Helps a lot, I used the magnet tool to extract spark plugs after they were loosened from their holes rather than try reaching my hand down into the coil well.
I'll think of more later