It should fit mate..
It may touch the linkage with the swingarm hanging down lower than it would in use. Did you bolt it all up and see if it came together?
Here's a 2012 with stock Ohlins 888 shock:
http://www.gasgasrider.org/forum/showthread.php?t=14036
I just don't see GG revising a shock from 12 to 13, before going to the purple Reiger shock in 15.