You have probably noticed that old maps like subtransit stopped working when Garry's Mod was ported to the OrangeBox engine. Sgt.Sgt, the mapper behind RP_Evocity2, has just found the solution we've all been looking for. View the thread for more information.
All you need to do is add the following parameter to the command line of games like Garry's Mod:
+r_hunkalloclightmaps 0
For games:
For servers:
Remember: You need this to run the upcoming map RP_Evocity2![...]