Im guessing with the little knowledge that I know of electricity, your fan, on start up, is drawing a lot of current. The GFCI tripps because it thinks its a short. If it were me, I would make another outlet next to it that is not GFCI protected. make a jumper from the line terminals on the GFCI to your other regular outlet.

OR, if you know how your shed is wired, and there is another outlet that is GFCI protected, then switch the outlets. Depending on how your shed is wired, you may still have a GFCI protected shed, while having an outlet that bypasses it.

hope this helps.