This article will walk you through uploading addons to your game server manually over FTP. We recommend using FTP over the File Manager almost always as it's more reliable and you can upload folders to your server instead of just single files.
Downloading the addon
The hardest part about uploading addons is finding an up-to-date version of the addon. garrysmod.org is generally outdated, so we recommend avoiding it if it's possible. Your best bet is most likely the Facepunch forums, inside the Gamemode & Addon Releases section.
Uploading the addon
1. Connect to the FTP server. If you don't know how to do this, please check out this guide: https://www.xenonservers.com/clients/knowledgebase.php?action=displayarticle&id=80
2. Download the addon to your computer, somewhere you'll remember such as the Desktop.
3. If the addon comes in a .zip, .rar, or .7z file, extract the files to your computer.
4. Check the name of the (extracted) addon folder. If it has any capitals in the folder name, such as M9KWeapons, rename it to be all lowercase. This is important since our servers run on Linux and there's a bug loading addons with capital letters in the /addons folder.
5. Upload the folder to your server's /garrysmod/addons folder over FTP. You can just drag and drop the folder into the /garrysmod/addons folder and it will start transfering the files.
6. If the addon has custom content with it (such as models. materials, fonts, etc.), then please follow the steps from the guide here: https://www.xenonservers.com/clients/knowledgebase.php?action=displayarticle&id=123
7. Restart the server so that the server can load the addon.