At the risk of exposing my ignorance-I'm trimming out others work-2-gang at sink counter fed w/12-3;cans were an after thought.pulled a switch leg into it-need other half for gfi-continues down wall 2-gang for fridge,then to low sg receptacle-power it up and the gfi trips-is it 2 hots W/ common nuetral that's the problem-switch fridge feed breaker off and it holds.

derater,
you can't share a 12-3 noodle off the load side of a gfi with another circuit. The CT will 'see' an imbalance & trip out.

Sparky's correct. I found this out the hard way a while back while trying to catch a steam shower can light off the GFI and leave the rest of the bath un protected. Ended up just letting everything go through the GFI.