Is it possible to edit gerber file using allegro?
You can do it, it's not going to be pretty though. There's an import option called import artwork.  After that you can make changes, but it's not going to take you back to the original database.  So it's not like you can import gerbers and then have the same experience of rats nests, nets, drc and the nice things you would have if you had the native allegro file.

But if you just need to tweak a few minor things and don't have / can't get the original database it can work.  Just not really recommended.  There are also other tools like Graphicode's Gerber viewer that can edit gerbers.  Fab houses use tools like this all the time to correct minor mistakes their customers make, or to add logos, change widths, do panelization, etc.

Not many people would use this approach to change connectivity or functionality though.