I am trying to get a map that I have in Unreal into GTKRadiant without having to completely rebuild the map. GTK only takes .map and .xmap files and in Unreal it does not allow you to export as one of these. I also can not find anything when I google this. If someone could help, it would save me a huge amount.

You’re unlikely to find anything useful - both use proprietary file formats and conversion between the two isn’t going to be something of interest to most people.