I keep getting told to use proper drum software, not Guitar Pro 6 for recording drums.

What I want to do is use the GP6 drum tracks I've already written, export them as MIDI, load them into some good-sounding drum software, and export the resultant audio track to replace the GP6 drums in the song master.

I just need to know the easiest software to do this.

I've never used EZDrummer or anything like that. Are they all only usable as plugins in a DAW or can some of them run as standalone applications?
Check out Direct Midi to MP3 converter. I've used it several times to convert midi drums and bass when not happy with GPro RSE playback. It's kind of drum machinesque (especially with toms and snares), but that's just my opinion. Anyway, it's very easy to use - just load up the midi and it converts it into MP3 'audio'.