Gender’s an overloaded term. Are you talking about the internal feeling, the way someone’s treated by others, the shared sense of a variable that differentiates people, social institutions, ideas, or something else?
Those of course are all related very strongly, but they’re not the same thing. Different models of gender will define of differently, but that’s usually just to best fit the area they’re applicable to. If a philosopher tells you gender is a social construct, that’s because they’re analyzing things through the lens of social construction. Very useful, but merely one perspective.
You don’t have to understand it, but you do have to accept it. That’s not “haha, unless”.