In order to get this girl to get with me, I'm going to tell her that my straight friend is gay.
That's not cool, is there a reason behind wanting to take this girl from your friend?
She's attractive and I like her.
Regardless of what you want, it's wrong to mislead this girl. You're also being disloyal to your friend, which is shameful. The girl can choose who she wants to be with, if anybody, without being manipulated.
I'm more attractive than him anyway, so it wouldn't be hard to get her to fall for me. I guess she doesn't need to think he is gay.
It's upsetting that you don't show loyalty to your friend, as that's something that is expected. It's good you are not going to mislead the girl, but overall you need to be more considerate towards the feelings and emotions of others.