Bicycle Tutor

I am in the process of giving my GT Aggressor a bit of t.l.c. I wish to replace the old rear mechanism with a new 7 speed device. Now do I buy a 'freewheel' or a 'cassette'?

How does one check?

Regards.
Bruce

Freewheel or cassette: http://sheldonbrown.com/free-k7.html

however, a 7 speed cassette can also be hyperglide or uniglide. Usually a freehub body replacement can fix that, though. http://sheldonbrown.com/k7.html

What you have is a bit difficult to say, since the GT agressor has been built over a decade with of course changing specs. Using the information on the pages above you should be able to find out. It basically boils down to "What moves when you coast / pedal backwards?"