All of my friends who have given birth in the seven years I have been here in the USA (Cnnecticut) have done this. It's very popular around here, and very 'American' I guess.
Personally, I'd prefer not, but I get cake (with a blue/pink inside, although two of my friends went for green for a boy, yellow for girl!) and it's nice.
Also, I don't seen any problem celebrating (unless it costs me money). My friend gave birth five months ago and had a gender reveal party a week after her scan. We came around, we just had general fun and gave some little cheap presents (baby stuff and maternity and pregnancy stuff) which we'd have given away, watched a movie, and our kids all played around (and my teens watched, bored) and we got a cake which was beautiful (in the shape of a rainbow and iced prefetch as one...she designs and makes cakes for a living...) and we got a pink inside.
Personally, I wouldn't have it but it was just a nice celebration for an excited first time mum? I think Americans tend to celebrate little things more (which is overwhelming for a humble Swede like me!) but the gender CAN be a big thing for a parent and they want to share it with us. A wedding us about, firstly, getting married, but you invite guests to share your happiness too.
I am happy for my friend when they have babies, I am happy that they are excited to know the gender. All the ones I have been too are like a normal group of friends coming over with kids, the kids play/moan/teens play on IPad and watch TV and MIGHT talk a bit, we chat together and talk, and we have some nice food together like normal, only difference is that we have a dessert which tells us something very exciting.
Personally, I have always gone 'oh, we had our scan, we'll be having a girl...at least we can reuse the old stuff from the DDs...' because I'd like to tel my friends (because they ask me questions and I ask them questions because we are friends and interested n each other) but no big deal tbh.
But some people like it. As long as they don't g overthetop, or mean I lose money, then it's fine.
It was also useful as it gave us a chance to get some baby things together beforehand so she didn't have random friends coming at random times to give random gifts just before or after the birth.
I agree it might seem showy offy but this was the only thing. None of my friends had baby showers, all of them were generally not over the tip or anything, it was just a day to celebrate them and their pregnancy and a chance fr them tell us news which is important to them.