I am not familiar with gameguru assets, sorry. I had to change my export assets in 3dsmax to get the proper path.
What you might try is opening your .MTLs in a text editor and manually adjusting the path.
You should see entries in the MTL that look like this:
Ka 0.000 0.000 0.000
Kd 1.000 1.000 1.000
Ks 0.000 0.000 0.000
Make sure the
map is just the name of the texture, and that texture exists in the same directory as your .MTL and OBJ.
If there is a folder structure present in the map path, delete it, just leaving the texture name.