Try a fairly heavy load that's compact and highly well insulated from ground - just hot and neutral and well insulated with nothing for it to leak off to anywhere nearby. If it still trips the GFCI outlet, you probably have a faulty GFCI outlet. Various intrusions can cause them to fail, e.g. moisture, ants(!), ... also possible, though less likely, that the GFCI circuit itself might be faulty, or there could be some wiring issue that's causing it to trip (e.g. by causing excess heat build-up from poor connections or faulty components).