Is it Possible?
 

Firstly I was unsure whether or not this was the right forum to put this thread since it also somewhat ties in with mapping, if I am in the wrong forum, Sorry.

I would like to know if it is possible to convert a Multiplayer BSP into a Single Play MAP so that it could be edited in Gtk and played in single player rather than multplayer.

Not unless you have the .map file.