I have all my factory rally gages working in my GP with my LS swap (minus the speedometer since I have a GP nobody has worked out how to put a electric speedometer in the stock gage cluster yet) monte's and malibus can use a 90ish suburban/G20 van speedometer that bolts in.
I use all the factory plugs on my 01ish harness minus the coolant temp, where i just used a different plug
Coolant temp- 3 wire 98 only firebird/camaro sender and plug, have to add a a 3rd wire since the ECM still sees 5V in and signal out, the 3rd wire goes to the gage in the car, newer LS stuff feed the signal digitally through the BCM
Tach- dont need an adaptor, just find the wire off the harness and connect it to the G body gage, hptuners is needed to reprogram the "out" signal to get the right pulses
Oil Press- use a 99-03 stock pickup sendor, it's 1 wire. I just found the wire in the harness where it comes out (one of the plugs you end up cutting off) if you use LT1 swap, and connect it to the G body gage.
All the wires come out of the bulkhead under the master cylinder. The plug you have is the one that goes to the gage cluster which you shouldn't need to touch if you have the engine compartment side harness.
What you need is the engine compartment bulkhead wiring diagram. You connect that with the LS harness under the hood.