I believe it is a torx T-10 size.
The hard part is the phillips screw on either side holding the speaker in. Hopefully you can find something to get in there. What I did, in a pinch, is to put a bit into a socket and turn it with a pair of vise grips to start the screw. They should come out by finger force after you get them started.
If buying GM replacements, the wiring will be the exact same setup. But, if you do decide to upgrade (why not?? factory GM sucks anyway) you will either need a plug-in harness (from best buy or whatever install shop) because the leads on the GM speaker and on upgrades are totally different.