@Hudson Been getting these same errors. The only thing that is not working for me are weapon packs #1 and #2. Everything else like models and such are fine.
Before you ask me, well did you do this... trust me, I've tried literally everything. I have 8,000 hours in Garry's Mod and have ran into many problems in the past that I've remembered every step to take when trying to fix something.
Not loading addon '[SR] Weapons - Content #1' - Invalid filename: materials/models/weapons/w_models/dlt19/m�ndung.vmt, �
Couldn't mount file [C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod/cache/workshop/915827513477443282.gma]
Not loading addon '[SR] Weapons - Content #2' - Invalid filename: materials/models/weapons/w_models/dlt19/m�ndung.vmt, �
Couldn't mount file [C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod/cache/workshop/915827513477451521.gma]
Failed to load sound "weapons\dc15a_fire.wav", file probably missing from disk/repository
AddMultiDamage: g_MultiDamage.GetDamageForce() == vec3_origin
I've tried uninstalling the 2 weapon packs and doing menu_cleanupgmas in console, after I deleted app_workshop4000 in my files and restarted Steam (for those who don't know, its the manual way to update an addon if it fails to update itself) and that didn't work.