Should I go on tinder while my gf lays next to me sleeping?
That sounds pretty messed up to go on Tinder behind your girlfriends back.
I want to sleep with someone else
If you want to sleep with someone else then maybe you are just not happy with your current relationship and you shouldn't lie to your partner because it is a horrible thing to do. You shouldn't cheat on her.
Just because I am having sex with someone else doesn't mean that I'm cheating on her.
That is the literal definition of cheating, unless you have her expression permission to do so, you shouldn't sleep around while you are with her. Maybe talk to her about it if you are not happy with your sex life or break it off if you can't be loyal to her.