Have a GFCI that trips when the ground and neutral are connected to the breaker and no phase wire
That would indicate that the neutral you are connecting is either not the neutral for the circuit that you have not connected the hot conductor(s) for, or is connected (improperly) to another neutral elsewhere.
GFCI's trip on a difference between the neutral current and the hot current [or the two hot currents, +/- the neutral given it's a two-pole].
Thus, there is current flowing on this neutral, (and not on the hot conductors which you have not connected) which there should not be if it's wired correctly, so it's not wired correctly.