Hi, this might be a very common issue, but I have noticed that, since the new texture pack, it's not working. I have used 1.12.2, 1.11.2, etc. I have removed the texture pack in the server texture pack folder in my computer, and have switched from Enabled, Disabled, and Prompt in the server texture pack settings. If you have come across this issue before but fixed it, please let me know.
Check out this forum post: https://grandtheftmc.net/threads/how-the-resource-pack-works.6100/ Moved to correct section.
Also, about the new texture pack, it was needed for a very cool feature we somewhat recently released. I am not sure if you know this, but you can now apply SKINS to your guns! You can achieve this via the skin crate which is at spawn with the other crates. For more info on this, you can always ask us for help via /help.