I've had this similar issue before but we found a solution that also thankfully worked for them
Opting in and out of Gluapack does not (always) work. Instead you should remove these folders:
GarrysMod/bin
GarrysMod/garrysmod/cache
GarrysMod/garrysmod/download
GarrysMod/garrysmod/downloadlists
GarrysMod/garrysmod/lua
Probably more than necessary but it seems to do the job.
Make sure you are opted back into Gluapack by putting this into your launch options:
If this still doesn't work then it might be a Gluapack CDN issue with our host. I'm not too knowledgable on how it is set-up but it might be an issue with Cloudflare as we use it for our servers.
Give Cloudflare Warp a try which also helped me as my ISP had a...