Wiring a light fixture in bathroom attached to a switch and GFCI
When wiring with cable, and the feed is coming to the light, the standard is to feed a single pole with the white wire (the code requires this) and return with the black wire.

If you connect the white wire to the other white wires at the light fixture you create a dead short when the switch is closed this would trip the GFCI if you are downstream from it. Check that the conductors for the switch are not dead shorting the circuit.

A picture or diagram of your particular situation might help if the previous tip doesn't.

Good luck!