GFCIs trip at 4-6mA of current NOT returning through the neutral. VFDs leak more than that as Common Mode noise. As mentioned, filtering often works, sometimes just adding a shielded isolation transformer works, I've heard people claim that adding ferrite core CM chokes will do it. But alternatively, a Class B GF device usually works too. But if you are using a VFD in an area that requires everything to be protected by a GFCI, take note that a Class B is NOT a GFCI. The term GFCI is SPECIFIC to being a Class A device. The others are GFEP (Grounf Fault Equipment Protection), not the same.