I have been given an unwanted gmail address which has effected by internet. How do I delete gmail?

Then you're going to have to actually explain what your problem is.

You want to delete it from where, exactly? And how were you "given" an "unwanted gmail address"?

Your post doesn't make a whole lot of sense.

See: Remove email accounts in Mail on Mac

Go System Preferences >> Internet Accounts. Is Google Mail Listed on the left panel ? If Yes, Remove it.

OK and good luck with your issue