Hi, I have an electric range and I want to convert to gas. Then gas pipe is underneath in the basement. Do I need a permit?

To do what ?
You'd be well advised to employ a professional when dealing with either gas or electrickery.

Depends on what's required where you live and how legal you want to be.
Call your building inspector and ask.
Plenty of people feel that they know enough to safely make new gas connections to existing gas and do so, but strangers online aren't going to guess about what you know.

Some areas require a permit with gas, some with electric. Your local permitting dept. will tell you. Most non big city locations do not need a permit. Are you comfortable with doing gas piping? Read up before considering, pipe thread fittings, pipe dope, gas shut off in the immediate area of the range, no compression fittings, flare fittings only.

There is no one in Ontario who can do a gas hook up except a licensed professional. Who would want to fool around with a gas line not knowing what you are doing!

I know this may not be in Canada, but who cannot help but call a gas company to find out no matter where the live?