We make this fairly easy.
1. Go to the Easy Setup tab in your Garrysmod NFoServers.com control panel
2. Under Garrysmod-specific settings, at the dropdown box containing "(choose install to put on the server)", select "Custom"
3. The SVN URL is the repository address to the gamemode or addon. I found a few of them here: https://sites.google.com/site/gmodsvnlinks/
but there are others. As an example, I'm using DarkRP, so in this box, you'd put "http://darkrp.googlecode.com/svn/trunk/" (DarkRP is also available as a non-custom option, of course.)
4. In Type, we'd select "Game mode", because, well... it's a game mode.
5. At Folder on server, you will put whatever you'd like the folder to be named that you want this gamemode/addon to go into. The system already knows it's a game mode, so you just put the name of the immediate folder. We'll call it CustomDarkRP.
6. If you're wanting to immediately use this custom game mode, we can go to the top, to "Default game mode:" and change it to CustomDarkRP, which is the folder name in which the game mode is located.
7. That's it! Go on down to the bottom of the page and hit "Commit changes" and the system will start downloading the files.