I replaced the seal on the pipe from the well to the pump. The tank is full of air, and I think the water in is still in the pipe to the footvalve. If I remove the pressure gage, how much water is needed to get the pump primed? Do I need to let the air out of the tank?

Do not bother the tank, other than making sure the pressure at the top valve is set at 2# below the cut in pressure. Might as well check that now that the system is, in effect, drained.

Simply pour water into the opening at the top of the pump (where you unscrewed the gauge) until it it full, then replace the gauge.