On Wednesday, June 12, 2019 at 9:27:04 AM UTC-4, wrote:
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

I have a Little Giant on a GFCI receptacle. That receptacle is on the same
circuit as the furnace. Ten years, no trips.

He could try plugging the pump into another GFCI. If it has to run to trip,
could put it outside, run a garden hose very slow into it, let it run
for awhile, see if it trips. It's not clear if the furnace is on the GFCI
too, would be unusual, but if so, that could be what's tripping it, though
it really should not if it's not defective.