Had the rocker cover gasket replaced as the mechanic said there was a leak. Interestingly I've never had a leak onto the floor but I have one now, only noticed in the last couple of months but I would say it's probably as a result of the gasket being worked on. I had the gasket changed 6 months ago and it's possible the leak started soon after that. Now I never checked the work as I didn't see the need to but now I have a leak on the garage floor so what should I do? Is it an easy fix to take the cover off and re-apply the gasket?

Should have mentioned that I have a Magna 2000 TJ Advance.

Don't see why you should have to, seeing it is only six months old. Sloppy job by the mechanic perhaps, take it back.
To answer your question - front is easy, rear is hard.