That's exactly what I was going to suggest. The error message is the same message you get when you try to use an entity that's been copied over from a standalone.
All files in standalone games (except audio and scripts as far as I remember) are encrypted to stop people from stealing other peoples work for their own use.
Try going to gamegur\files\levelbank\testmap and move the file _e_vegmask.dds somewhere else, then go into steam, right click gameguru in your list, select properties, go to local content, and click the verify files button. Then when it's done try running gameguru again.
i5, NV960 2GB, 16GB memory, 2x 2TB Hybrid, Win10.
i3 , Intel integrated graphics, 6GB memory, 512GB Generic SATAIII Win8.1.
Intel Celeron (duel Core), Radeon integrated graphics, 4GB memory, 180gB Generic SATAII, WinVista.
Q6600, Intel integrated graphics, 8GB memory, 512GB Generic SATAII, Win7.
