Welcome. My name is XXXXX XXXXX would be glad to assist.
Sounds like all the wiring
is in place, so lets look at where they connect at the disconnect. and tub.
You have 4 wires from the disconnect to the tub also, correct?
2. Look at the small block in the disconnect that you connected the whites to, I do believe that is the ground block. Your other electrician is in the right direction it sounds.
Wire nut the 2 whites together inside the disconnect so the white goes directly to the tub from the breaker with no connections between.
Take the ground wires from the panel to the disconnect and to the tub, connect them to the small ground block in the disconnect.
Make sure all connections are tight at all locations including the tub.
See if that doe snot take care of the issue.
GFCI's are very sensitive.
Just hit reply after the connection changes and post results, but that should get it going.