Can I use my phone as a bluetooth game controller for my computer?
You haven't mentioned which types of games you need this for but there are several possible options:

Gmote 2.0, which is a client/server type system which would let you use your Android tablet to control your computer.

RemoteDroid, which is similar to Gmote, it also uses a client/server system.

GRemotePro may be the right choice for you as it is but you need to pay for the server software. It is specifically advertised as being for game control.