Simple question about converting 240v to 120v

Hey guys, need some advice, please... this is probably a dumb question but hey, want to make sure.... I am going to replace an electric cooktop that is 240v to a gas cooktop that runs in 120v for the "minor" things. Anyway, since I don't need the 240 jbox (no proper outlet but it is under the countertop, inside a cabinet) I was thinking in reusing the cabling for the 120v and make a simple outlet that I do need. I think I have a 3 wire cable for 240v so technically speaking will be just CAP the red (in both ends), replace the breaker for a 120v 20a and put a cover in the empty whole, Only connect the black to the breaker because the Neutral (white) and Bare (ground) are already connected to the bar... is that simple? am I missing something? Does it need to be GFCI? Can I use a new metal box or does it have to be in wall?

Always good to doublecheck with the pros and thank you for helping us normal folks clarify some electrical questions