For games that have a Steam Workshop, Garry's Mod first and foremost, the panel integrates it directly: the game catalog can be browsed from the tab, and a mod is added with one click.
The catalog, and adding by number
Search queries the game's Steam catalog, with the number of subscribers as a guide: an addon followed by hundreds of thousands of players has proven itself. Each item can be added with a button. And if you already have the addon in mind, paste its number or its Steam link into the field at the bottom: it is the shortest path.
What you need to know
Additions and removals take effect on the next restart : that is when the server downloads the files, and a large mod can lengthen that startup by several minutes, especially a large map. Add things in small steps and restart in between: if the server does not come back up, the last addition is the likely suspect.
Games without Workshop do not show this tab; some have their own mod system, managed from their settings or their files, and the panel adapts to the game installed. If in doubt about a specific game, support responds.