If I can be Real for a second. I think one of the reasons I always write Gemma and Thomas as a cis woman and man—despite heavily fw-ing with multiple trans headcanons, rotating them from time to time, and adoring sapphic gemmas—is because, at the end of the day, I'm 1) very Gemma-biased between them two, and 2) Gemma's whole core narrative trait to me is that her story is a deeply familiar (horror) story to any woman. Let me explain.
Single parents are overwhelmingly mothers. Men abandoning their wives/girlfriends/lovers and the child they have together is so stereotypical and common that we have age-old jokes of dads going to the store to buy milk. Even in married households, childrearing very commonly falls on (or is expected to fall on) women and not men.
And Gemma embodies them all so well, these unfair expectations on women. It's such a familiar story; think about how much more unexpected it might be if Thomas is a woman who talks about her ex-husband in the first scene, and then Gemma comes onto stage as a man being snappish that "no, you can't go and see your mother, because she's joined a fucking cult!" I know I would have a huh moment!! Thomas can leave his family behind so thoughtlessly and recklessly (and it absolutely was thoughtless and reckless and inconsiderate and a thousand other negative adjectives), and I'm wholeheartedly convinced that part of it is because of the natural, unquestioned, socially accepted assumption that the child will be fine if left with his mother. Everyone assumes that a woman—a mother—will have all of childrearing figured out the moment she becomes pregnant, because motherhood is sacred and and wonderful and so magical and clearly every woman is meant to be a mother, and it's so all-consuming that women's identities are often shrunken down to only mother (and wife). Hell, Gemma's whole narrative identity is being Thomas' ex-wife and Tommy's mom!! That's it!! That's fascinating to me!!
(Note: this does NOT mean sfth is misogynist—they are not. This ^ played out in this manner because Gemma's narrative function is a supportive character whose existence is defined in relation to main characters, and the protagonist turns out to be Tommy (and to a slightly lesser degree Thomas). We cool? We cool.)
And in what relationships are single mothers most commonly found, by virtue of being the most common type out there? Cisgender heterosexual ones. The gender plays such an important role because the story is familiar to everyone, and it's so overwhelmingly one specific subtype of gender (cis man) doing the leaving, and so overwhelmingly another specific subtype of gender (cis woman) being left behind and unfairly expected to shoulder a burden too big for one person and do just fine. It's much harder for me (for me only!!) to enunciate the gender politics at play between them and informing Gemma's situation, if Thomas is not a (cis) man and Gemma not a (cis) woman.
Also ig reason 3) Thomas is absolutely the villain in this situation, and I really don't like writing a dynamic where the trans person is at fault for something completely separate and just also happens to be trans, and then they're the only person in the whole work who's trans. It gives too much room for some icky explanations to be possible ("well she did that because she was socialized as a man!!" No).
(This also isn't to say trans/nb people can do no wrong ever. Source A: I'm plenty nonbinary and plenty capable of being a real cunt. Source B: Magnum O'Puss ig. But hopefully you get what I mean here.)
(I also don't think Dillon is trans, but that's for another post. Gemma can be trans/nb tho. So can Leila. And Tommy as he grows up. But once again I hope you get what I mean here.)
Anyway idk much what I'm trying to say, but. Nods at Gemma's direction. Unfair expectations of motherhood and wifehood on women yeah. Luke Manning used more his normal/real voice as Gemma than a "female" voice yeah. Luke as Gemma grounded in reality yeah. Me also trying to ground it in reality yeah.