If the decals don't work, here are the reasons why and how to fix them.

1. You may have an addon with conflicting decal replacement files. Unsubscribe from the addon and delete the files from your garrysmod/garrysmod/addons folder.

2. You may have decal replacement files hardcopied in your garrysmod/garrysmod/materials/decals folder. Simply delete them.

3. You may have to restart Steam. This is the most common issue when decal addons, they often require a Steam restart. That's all.