cant open games on steam

[Terminatorwath]

i just created my own gaming pc, and downloaded Garys mod, but when i attempted to open the game it shows the "preparing to launch" screen then closes. how can i fix this?

 

[BossieBanks]

Go through your steam files and click on common then find gmod and open it through the .exe and it should tell you if you're missing any .dll files or anything.