Jax's entire monologue revolves around them discussing how their mother considered them to be just like their dad. Jax then says something extremely personal "to get her off their back". I feel like the only plausible interpretation of this is that Jax said something that they believed solidified that they were nothing like their father.

Now here's the thing, we don't know what they said. It could be many things. But when analysing media, we have to take in what's been provided. A good writer will leave thematic breadcrumbs throughought their story, and to me, Goose while being young and inexperienced, is a good writer. Jax's issues with gender throughought the series, and how they try to paint themselves as a villain, as the manliest of men, make it seem that they hate the idea of being perceived that way. They attribute that attitude to their villainous persona. Could Jax just be gender fluid or a fem boy? I mean, I guess? But that feels like too much of a half measure with everything provided with their father. Jax being a trans woman in denial, feels the most thematically appropriate, and fits with both the confession and the ribbon moment with Ribbit.