Mansplaining is when you explain a subject to a woman as if she doesn't know it, when she would be fully expected to know it. An example would be a man without an astrophysics degree explaining astrophysics with condescension to a woman with an astrophysics degree. It sounds silly but I've seen it happen, more often than not it happens online though because terminally online people tend to be more condescending.

Manspreading is more often when dudes intentionally take up more sitting space than they clearly need in public when it's obvious there is enough space for additional people. Often it's a lack of self awareness.

Male gaze is the way women are most often portrayed in visual arts and media from a heterosexual, masculine perspective, often objectifying them as sexual objects for the pleasure of the male viewer. It suggests that media is often constructed and consumed from a male perspective.

Hell, even a lot of sexualization of men is from the male perspective. Having spoken to a lot of women about how they experience attraction, most aren't very interested in the hypermasculine view of the male "ideal body" and are far more interested in certain behaviors and mannerisms, or even just the look of their face and hands, rather than everything else.

I'm writing this not as an argument, but as taking your questions in good faith. I hope it was in good faith.