Why... when I replaced a regular outlet with a gfci and the switch won't shut off the garbage disposal? The outlet is at the end of the circuit with a red, black and a white wire. I can get power to it but won't shut off. It worked with the regular outlet.

You replaced the standard outlet that the disposal was plugged into with a GFI outlet, and now the disposal runs continuously? Sounds like you connected hot and neutral to the GFI outlet, rather than switched and neutral. Please describe (a) the wire connections that used to be on the old outlet before you disconnected it, and (b) the connections you made to the GFI outlet.

Yes, agree with ebaines. With a red and black, you have a split neutral. One of the wires was switched, the other not. You should not have a red and a black on a 120 VAC outlet. Remove whatever hot you used and trade it with the other colored wire. It should be on"Line".

Take the GFCI out. It is not needed in your instance. The code does not require under cabinet garbage disposal receptacles to be GFCI protected.

Please tell us that the disposal is not wired to any countertop receptacle!

That would be a violation of the electrical code.