For some people i've noticed they cant see the textures, I have a small fix that might work.
For people that can't see the materials of the crystals (model covered in pink and black checkers)

Start by Un-subscribing from the addon and Verifying Game Cache.
Right click on Garrysmod with using (Steam Browser)
Then click Properties > Local Files
Then click "VERIFY INTEGRITY OF GAME FILES".
Let it finish checking for missing content in garrys mod then go back to the "Crystal Cluster Prop Pack" and Re-Subscribe Click here.