I'm bothered by my girlfriend wearing other guy's clothes
Why does her wearing other guys clothes bother you?
She is my property, so it makes me uncomfortable.
People are not property. Even if it makes you uncomfortable because she's a woman, it is her choice how to dress. You can tell her how it makes you feel, but it is up to her what she wears.
I'm going to threaten her by telling her I'm going to cheat on her then kick her out if she continues wearing guy clothing.
I do not think threatening her will help. It could hurt your relationship. Just let her know that it is bothering you, as this is really the only way you might be able to overcome how you feel.