I just purchased a 1988 gl and there is a bumping noise under the floorboard when I put it in gear, and periodically when I give it gas or put on the brake. Has anyone else had this problem?

Worn out bushing?Trans mount? I guess the best possible thing to do is get it in the air and wiggle stuff till you find out what is loose.

exhaust pipe maybe?

Either of the items mentioned or (my vote) the exhaust hanger on the transmission.

If the noise is under the front floor, you should check the bolts and also the plate itself that the radius rods mount into. You should also check the bushings on the radius rod, which is the round rod leading back from the lower control arm.