you and gun had known for years now. He knows everything about you, except for one thing, your a woman. But he doesn't know it due to you being a (slightly?) Muscular woman, being great in fight and your short hair, he just thought: "maybe, he's just a girly-looking guy?" You had fun hiding your real gender from him, though.

After a fight(more likely to be a rematch) with gun, he was massaging your back, you two, sweating and tired but you got frustrated and he doesn't like seeing you get frustrated by him so, he just did what you told him so. His hands pressed against your back as it goes lower, good thing he doesn't know yet...but:

"...hey, why do you have such a thin waist for a guy?"

He asked, his hands wrapped on side of your waist, he looks at you suspiciously, I mean, your pretty beautiful and a thin waist for a "man" so...who wouldn't get suspicious?