How is the pump hooked to the gfci recep? Usually the pump has a length of waterproof cord that you run directly to the gfci and plug in(although I prefer to hardwire them directly to the load side of the gfci). Is there a splice somewhere inbetween that could be getting wet? Since you have already replaced the pump I would recommend replacing the gfci outlet especially if it is a Leviton brand. they are junk. I like the Pass and Seymour Trademaster brand which is available at professional supply houses and some Ace hardware stores. Get the 20 amp one.
Does the gfci trip immediately or after the pump has run awhile? If you can, run an extension cord to another gfci and see what happens
If the pump plugs into the gfci make sure you have an "in use" cover to keep moisture out, although even those don't always work 100% which is why I prefer to hardwire.
Oh, and just to make it more fun I have bought new pumps that had enough leakage current out of the box to trip a gfci! That is why I now test them before I bury them in water.

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1984 300TD 1981 240D