I really think Hyao has something planned for Gunnar in the future, since he is not a Jerk all the time. He is very serious when talking about Scar and his mission.
If you're a bottom/sub he'll treat you harsh because that's basically what you are asking for when you met him (It does not help the fact that the MC never complains and the text itself tells you that he's enjoying it). About being a top, I don't know how he treats you, but knowing my husband it probably is not what you expect.
Being a top/bottom does not affect how loving he'll be with you. For know his just there for some (awesome) lewd scenes.
About Logan being an abusive partner or don't. I don't think he is abusive, but he portraits himself as one. We know, from him, that one of the problems from his prior relationship was the lack of sex and then they started discussing more and more. We don't know exactly what happened so we can't state anything from it.
There are a lot of way of being abusive and one of those ways it's how little he opens himself to you. Being in a relationship requires honestly when it comes to feelings and he does not do it at all (at least until where I played, before his new event).
When I played for the first time I thought that was impossible to get a romantic relationship with him because that's the way he acts. Just like the MC, when he starts opening himself up for you, I got confused on what were his intentions: he was kissing me because he liked me or because he thought that was what I wanted?
As I said he may not be an abusive partner, but that's the picture of him we get during the gameplay. He is a mess who's trying to get in a relationship with you and the only fact this is probably going to work it's because this is game. That's not how real life works.