Oh, I'm sorry, just re-red my reply, wrote it a wrong I think. I mean, if I tab out of the game (Alt+Tab), the taskbar is not there anymore but I can see the game through it.
And yes, it is seen as a fullscreen application:
FullScreen Applications [1]
• 0x000307d6 ~ {X=0,Y=0,Width=1920,Height=1080} ~ FillsOneMonitorWindow ~ "Garry's Mod" ~ "Valve001" ~ "C:\Program Files (x86)\Steam\SteamApps\common\GarrysMod\hl2.exe"
That's how it looks, when I tab out:
http://puu.sh/d4LLm/3462245081.jpg
OK, I think it's a "bug" (or maybe a feature?) of the Taskbar-Magic because when I move my mouse to the far bottom, the windows taskbar comes up but without the windows circle thingy oO You can also see a bit of the taskbar...
Pretty sure I haven't had this issue yesterday, restarting computer, maybe just a strange glitch or something.
Edit: OK, looks like it didn't execute Taskbar-Magic, don't know why...
Now, when writing the edit it came up randomly again when I moved my mouse to the bottom but was instantly gone again..
Edit2: Just read the readme of Taskbar-Magic, the 2nd thing is a known bug there it says:
"Known bugs
[...]
-Sometimes the taskbar likes to come back (like if you invoke the start menu), just wait <2.5 seconds, and my program will hide it again"