gfci outlet to switch for garbage disposal
I do not think you did it correctly but I will explain. 

Connecting the GFCI without a switch all sounded good. But you then put the switch on the white or neutral if I understood your question. 

The proper wiring would be the “splice” to the switch if a piece of Romex is on the hot to the outlet , you remove the black line feeding the outlet connect that to the white of the Romex and mark that white at both ends with tape or a sharpie. That reidentified white connects to the switch, then the black in the Romex “splice” connects to the other side of the switch and at the outlet on the hot / line side. 

I think you switched the neutral side this is a code violation. 
We re-identify the white and have it always hot so someone later doesn’t mistake it for a neutral. But that is the proper code compliant wiring for a switch leg. (+ grounds and while the breaker is off but you know that).