A combo-amp is a preamp+poweramp wired to a speaker. All of this is housed in one unit, hence the name combo.
Amp heads are preamps+poweramps. They are as you might have guessed, in the same housing and needs to be connected to a separately housed cab.
As srob said, if you want a GP-1000 to work you have to get a poweramp as well as a cabinet. Connecting a GP-1000 to an amp and then to a cab would just create a big mess (don't even think it's safe).
Think of the poweramp as a really big and expensive volume knob.