In order to make a user an administrator you have to first of all locate the Configuration Files option in your server menu.
After that, look for "garrysmod/settings/users.txt" and press [ Text Editor ].
It will open up the file, after which you can add administrators.
Start a new line or look for the comment lines ( // ) behind "garry" and remove them to make the line usable.
Make sure the name is in quotation marks. You can use anything or leave it empty, because the only factor here is the Steam ID, you must able to get it first.
Have a space after the quotes and insert your Steam ID into a quote as well.
This should make you an admin or a superadmin depending on which array you've chosen.
The end result should look something like this.
