I am replacing gas hob with an electric hob how do cap gas supply

Capping the gas line is pretty simple. Just buy a cap that fits the line size that you have - and be sure to use some good pipe dope or teflon tape on it... then when you are done test for leaks using a thick soapy solution.

If you've not worked with gas lines before, you should have a professional do it so that you are sure it is put on correctly, and that there are no gas leaks.