i’m unsure what you mean by feeling ambivalent about gender. i got some ideas:

1. ambivalent regarding the concept of gender: the existence of agenders has already been pointed out. even aside from agenders, people can dislike the societal system of gender, think it’s "made up"¹, annoying or the like, and still wish to place themselves in the binary. it took me long to accept myself as trans bc of this ambivalence, but i hate being seen as a man more than beeing seen as a woman. (i’m a little bit of an enby maybe.) also theres gender fluid people for whom the supposed constance/persistence of gender identity may not make that much sense.
2. ambivalent regarding their own gender identity: if someone feels like they are unsure what they are, not this but also not really that. it might be that they are still not allowing themselves to fully feel in this or that way. repression made it hard for me to even see that there is a big part of me, that wants femininity. it could also be that they are gender fluid, enby or agender, and thats why they don’t have an "unambiguous"² feeling of identity.

¹ well i see the fun in being polemic, but english is also a societal construct much like gender: the signs and rules are arbitrary, highly conventional, and it has no individual architect. yet it is there, we can use it to communicate, enjoy, confuse, etc. … ‘made up’ implies that it is easily dissmissed from the conversation and that’s not the case i guess. sure there are different languages of gender culturally, but still. tgey exist and affect us.

² that’s a bad word right from the terminology and expectations of cis people. enbys, agenders and gender fluid persons are not beeing ambiguous when telling what they are. the predominant system of gender just doesn’t know what to do.

hope, that helped in any capacity?