I need help about Garry's Mod.Can you help me?

When I want to play Gmod
Steam prompt me: “No Steam Apps”
Why?

Do you own the game?

Of course

Prove it.

Do you own a Source Game?

how do I prove that?

[editline]14th January 2011[/editline]

Yes,I do

Link us to your steam account so we can see that you own it.