You know you've been playing GMod too much when...

[ixxassassinxxi]

When your car gets stuck in a ditch so you throw barrels and couches at it until it moves.
When your teacher talks to you after class about lack of performance you yell "METAGAME!"
When you own all the buildings in your town and they all have your name on them.
When you see a dog and yell "AAAH! ITS DOG!" and run.
When you go to buy a new computer you ask the clerk "Does this run PCMod 2?"
When you walk into the Best Buy and start looking around (standing still) you yell "freaking PING!"
When you see someone on a computer doing what looks like hacking you throw it at them and scream.
Whenever you see an old black guy you yell "DAD!"
When you think about Alyx more than your girlfriend.

hmm.... I do the last one a lot, so I better stop playing so much gmod

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When you sit and play Gmod more than 1 hour each day..

I've managed to rack up about five hours.
In one sitting.

Wait, here's another:
When you buy a game just so you can mount it into GMod, rather than actually play it.

I've done that lol