Garry's Mod Help

[WhatevaGuy]

I was trying to play Garry's Mod today.  (Error: attached)
What am I doing wrong here?  Do I need a different source game or something?  I asked somebody on HLDM: S and they told me to reinstall it, but I'm a moron unsure of how to do this.
Can anybody help?
Thanks a bunch.

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[yuki]

Start up HL2/HL2:DM (HL2 uses Source) then close out of it, then try gmod.

[IkeTheGeneric]

Too many people get this wrong and it's stupid

[WhatevaGuy]

Start up HL2/HL2:DM (HL2 uses Source) then close out of it, then try gmod.

I'll try that, thanks.

Too many people get this wrong and it's stupid

Oops, honest mistake.  Sorry.  I'll do better next time.

[WhatevaGuy]

Start up HL2/HL2:DM (HL2 uses Source) then close out of it, then try gmod.

Worked.  Thanks a lot.

I'll put this in the right section next time (although I hope there won't be a next time) :P
Locking.