Add gpsim description file.
Add gpsim description file.

TianpeiLuke/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,TianpeiLuke/GPmat,SheffieldML/GPmat