"Chucking a Gary" is term given to an individual who creates the most bullshit stories you've ever heard. A fabricator of the most elaborated and ridiculous stories imaginable.
Person1 “hey man, did you hear I’m paying to be the first space tourist ever?”
Person2 “Bullshit, stop chucking a Gary”
Person1 "nah seriously, and my uncle has 10 ferraris in his roof"