Removed an electric stove from a double pole 30A brkr and want to replace with a single pole 20A for a new gas cooktop. Must I remove the 10 awg or can I use it with the 20A brkr. I will put a cover inplace to take care of the opening from the double pole brkr I removed.

No problem. You can use a 20 amp breaker with 10 gauge wire. Cap both ends of the unused conductor with a wire nut. Probably easier to use two single 20 amp breakers (leaving one for a spare) than to find a blank cover to fit your panel.

If It had No Neutral, you can Identify both ends with white tape, also may need to pigtail a short Piece of #12 to attach to the recpticle. If you have a neutral, then you could have 2 20 amp circuits with a 20 amp 2 pole breaker and have each hot go to to separate recpticles, and connect the same neutral to both. Fills the space and gives an extra circuit. Its referred to as a "MultiCircuit"(2 circuits on opposite phases sharing a neutral)On opposite phases, the Neutral carries the DIFFERENCE of the 2 loads, If 1 is drawing 14 amps and the other is drawing 15 amps, the neutral will be carrying 1 amp. Research more or ask anything not sure of if doing a multicircuit. Good luck.

Meant to say identify 1 conductor at both ends with white tape. Then place on Neutral bar.