I don't think gender reveal parties are inherently transphobic, or even homophobic. I think it is more about why people do them, or why people care about gender in the first place. The reason people care about their baby's gender is because of the stereotypes that come with it. Maybe mums want a girl that they can put in pretty dresses and go shopping with when they're older, while dads want a mini-me that they can take to the football. Sometimes people even say you need to know the gender because of how to decorate and what clothes to buy - but then that also gives credence to the idea that baby boys and baby girls need to dress differently and sleep in rooms painted a different colour. So people take umbrage with gender reveal parties because they give legitimacy to gendered stereotypes.
Being gay or trans is always going to be harder if you have parents who really care about these things and wanted a boy or a girl for particular reasons. Trans people are inherently gender non-conforming obviously, and I would say most gays are in some regard too (even if just because not being into the 'opposite' gender goes against gendered expectations/stereotypes). Regarding gender reveals specifically - as a trans person, I imagine knowing that your parents had a big party to reveal your gender might be disconcerting as it demonstrates that they care about it one way or the other.
But honestly it is probably more important what they do after when the kid is there. If you have a gender reveal and then go on to be the sort of parent who says "no, you can't do x because you're a boy" or "no, you can't do y because you're a girl" ... that's the real problem and it probably indicates to that child if they are gay or trans that it is not 'safe' for them to be themselves around their family.
However - I think quite a few people who have gender reveals do it quite innocently for a bit of fun (any excuse for a party!) and aren't aware of what it signifies to other people (including potentially their future child) but potentially should be.
Even as a non-binary person I think Paloma's approach of waiting a few years and letting the kid decide their gender themselves is a bit "that one friend who is too woke". But as a non-binary person i've always been grateful for the approach that my parents took with me, which was that when I did get old enough they never tried to either force anything on me that was gendered if I showed resistance, or say no if I wanted to do something that was gender non-conforming. My mum always tells a story about me being about four and her putting me in a dress and me going upstairs and changing into leggings and basically saying I didn't want to wear dresses - and so from that point on she basically said she wouldn't put me in a dress unless I suggested it (which I didn't). I wore a mix of boys and girls clothes, I loved watching wrestling and going to the football with my dad, but I also played with Barbies and liked pop music and painting my nails. There was no active convo about 'gender neutral parenting' or anything - they just let me be myself. In doing so they also gave me the best possible indication that they would be pretty chill if I came out as gay, trans or whatever.
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