To be able to use a chipped ECU setup you would need an OBD2 to OBD1 ECU conversion harness, an OBD1 distributor, and of course a chipped OBD 1 ECU.

If your GSR has the stock IM with the dual runners, then you'll want to use an OBD 1 GSR ECU which will allow you to still use the dual IM runners. If you have an aftermarket single runner IM (like the ITR) from Skunk2 or Victor-X, then you can use an OBD1 P28. With a single runner IM you dont have to worry about dual runner support so you can save money by going with a chipped P28. If you do get a P28 you also loose the ability to read your GSR's stock knock sensor since P28 ECU's dont support knock sensors.

I agree with Das Schmoo though. If you only have I,H,E then it wouldn't be worth it to get a chipped ECU setup. But if you decide to get cams and gears then a chipped ECU would be worth it cause then you can start tweaking your setup to get a lot more power out of that setup.