I am replacing an electric cooktop with a gas cooktop. The electric cooktop is wired to a 30 amp breaker. The gas cook-top needs a 15 amp breaker. Do I need to pull the 30 amp service and run a new line from a new 15amp breaker? There is plenty of room in the panel to do this.

If you have 30 amps at cooktop, you can reidentify one as a neutral with white tape, and place on the neutral bar, you may need to pigtail a short piece of #12 or #14 to fit in breaker.
Then change receptacle for new cooktop.

The breaker just protects the wiring; it has no other purpose. If you need a 15 amp receptacle I assume you are going from a 240 volt to a 120 volt receptacle. Just take one of the wires off the double breaker and put it on neutral so you have a hot, a neutral, and a ground. Then change out the breaker to a 15 amp (not really necessary) and put the hot wire into it. From the gas cook-top you will have a black (hot), white (neutral), and green (ground). If it plugs in you will have to wire a receptacle in which case you will wire hot to brass screw, neutral to silver screw, and ground to green screw. It's very simple. DrTom4444

I wish I know about this.. but.. I don't sorry.