OP,

does the GFI trip

1) very soon after the pump is plugged in even if it is not running?

2) after a while when the pump is running

3) at the same time that the pump starts or stops.

If 3, it may be the noise surge when the pump switches on or off.

I think they make another version of GFI that is for higher leakage. You might try to find one of those.

Also, I think code allows (or did in the past) an outlet to be mounted high up at the ceiling to not have a GFI. This is intended for sump pumps.

mark