I don't think slagging off individuals or groups by calling them names or slurs is cool whatever the intent. IMO, most of the people who advocate "reclaiming" and repurposing slurs are puerile and unsophisticated in their thinking.

If a person's gnc behavior or expression lead to them being called homophobic slurs, would they be in the right to reclaim those slurs if they actually weren't LGB?

I think you are presenting things backwards. You're saying that being subjected to homophobic verbal abuse is the result of individuals' "gnc behavior or expression" (whatever "gnc" is supposed to mean). When, in fact, homophobic abuse is the result of the fucked-up, intolerant, sexist thinking of those who spout such abuse.

IMO, the onus for homophobic slurs and abuse needs to be placed firmly on the people who spout such slurs and commit such abuse, not on the recipients.

BTW, some people who have been subjected to homophobic slurs and abuse over time have not been either homosexual or "gnc" in dress, grooming style, mannerisms and other superficial aspects of appearance and affect.

For example, other people have used homophobic slurs pertaining to lesbians against me personally for more than 55 years. Mostly on account of my feminist ideas, writings, actions, outspokenness and friendships and alliances - not because I had a particularly "gnc" look or mannerisms (though by current rigid standards, the youth of today might view my "presentation" and behaviors in the past and now as "gnc.")

I also think it's relevant that some of the worst homophobic abuse I've gotten in my life has been from a few homosexual people. For example, in 6th grade (age 11-12) I was subjected to a severe bullying campaign for supposedly being a lesbian that was started and sustained by a female classmate and former close friend trying to deflect her own and other people's attention from recognizing that she herself was a lesbian. (Many decades later, she and I reconnected and she apologized.)

In recent years, a number of homosexual males and females and het and bi ones too who hew to currently-fashionable sex stereotypes have slagged me off for failing to conform to their sexist, ageist ideas of what "real women" should look like, sound like, behave like and think like. These people think that surgically-altered male people like Blair White, Gigi Gorgeous and Munroe Bergdorf are much more "real women" than I and my peers are coz these males have painted acrylic nails, long hair, "girly" mannerisms, they take exogenous estrogen, and they've undergone medical procedures to feminize their faces, remove their beards and implant orb-shaped sacs of fluid or gel into their chests.

Significantly, the homophobic verbal abuse I've had hurled at me has been dished out with no regard for my actual sexual orientation or pattern of IRL sexual relationships. At the same time, though, some homosexual males who ID as "trans" have called me all sorts of disgusting names because I've borne children.

I also don't understand what you mean by "LGB" here. Coz you seem to be endorsing the muddying of the waters that gender ideologues promote by saying

if a transwoman who was exclusively attracted to men grew up being called "f-gg-t" and things like that, would it be acceptable for her to reclaim that slur for herself? Some people would see her as a man, therefore gay, and some would see her as a woman, therefore straight.