My oil pressure gauge reads right about zero. Before i installed the tranny it read about 15-20 at idle and went up to about 40. I replaced the rear main seal, its not leaking anything. The gauge doesnt move while i rev the engine. Is my gauge messed up, how can i check it?

 

First thing I'd do is not drive it until you find out if it's indication only or an actual oil pressure problem. Pressure that doesnt change with RPM is usually bad indication, either a bad pressure transmitter, or guage (or biasing resistor for guage)
If you have a direct reading guage, you need to plumb it into the oil system, probably at the oil filter housing or where the oil pressure sender is plumbed in. If you have good oil pressure, check the pressure transmitter wiring & plug condition, it often gets burned from accidental contact with exhausts. Trying a new transmitter will probably set you back $40-50. After that it's probably the guage and I hate working on instrument panels...so I cant help you there....my T/A is still without an accurate oil pressure guage 'cuz it's the guage bias resistor & I aint gonna spend the money on having that fixed - I know actual oil pressure is good, so I drive it & dont worry.
cheers,
Ian