Garry's Mod Workshop Utility (Easy create, upload.etc).LuaWorkshopper - Quick tool for Addon making and publishing.Garry's Mod Easy Addon Uploader (for Windows only).Garry's Mod Publishing Utility - Java based GUI wrapper.gmpublisher (GUI for Windows, macOS and Linux).Links to such known tools: (feel free to expand) You can use alternative tools such as GUIs (graphical user interfaces) made by community members to create, extract and upload addons to Steam Workshop. Please see this page for complete set of rules: There are a few rules before you start uploading addons.
This article will teach you how to prepare and upload an addon to the Steam Workshop.