1. This wasn't a question, it was a statement. But the wires aren't there if the car wasn't originally equipped with the optional gages. You'll need the circuit board and harness for the gages you want to install.
2. Yes. It isn't necessarily trim level specific as much as gage option specific.
3. Yes, depending on your level of electrical/electronic troubleshooting and repair skill it is entirely possible to wire up factory gages with non-factory style wiring. But it has to be right, or you could have a flaming hot GP in a way you really don't want.
If this doesn't answer your questions to your satisfaction, you may want to try one or both of the following:
Poncho Pride forum dedicated to all mid-seventies Pontiac models.
A Body Site forum dedicated to mid-seventies GM A bodies.
Both of these forums are monitored by people with a lot of experience with the GP style dash, which was used not only in the GP, but also in the Grand Am, Can Am, and Grand Lemans as well.