Adjusting To 13
There's a few thing that aren't immediately obvious in the new version of Garry's Mod. These may lead some veterans to think there are missing features.
The Context Menu
To open the context menu hold downand right click on an entity. You will see a bunch of options, such as keep upright, make statue, edit bones.
Change Player Model
To change your player model hold downand click on the Player Model icon on the top left.
You can disable NPCs, make NPCs ignore the player and change an NPCs weapon by opening the spawnmenu, or holding down C - and selecting NPC from the top menu.
In the top menu you'll also find options to hide certain props and disable certain effects.
All gamemodes require a gamemode_name.txt file now instead of an info.txt. For example, the DarkRp gamemode would need a darkrp.txt file.
sv_gamemode no longer exists. You can now set the gamemode with the gamemode command. The command is case sensitive. Type in the name of the "info.txt" file which should now be changed to the gamemode name.
Downloading stuff from the Workshop
Toybox is gone. In it's place is now the Workshop, which can be reached from the main menu by clicking on either "Addons", "Demos" or "Saves". A list will load, which represent the files in the Steam™ Workshop. Hover over a file to see buttons which allow you to rate and (de-)install the plugin, demo or save.
Note that installing plugins requires a rejoin / reload of the map and in rare cases a reboot of the game.