I'm surprised at how many posters say "no gift" - I can't imagine showing up to a party that's celebrating a new child and likely serving me some food/drink I'm arriving empty-handed?
My understanding is a lot of people that have gender reveals do not have baby showers. Some do. But I think a lot of gender reveals are in place of that. Whether or not one agrees with the idea of the gender reveal in the first place, I think if one does attend, it calls for a gift and it's not difficult to bring a simple gender-neutral gift. Anything from diapers to books to clothing.
On a related note, if no gift is, in reality, the attendee's way of (passive aggressively?) asserting that they think gender reveals are stupid, or they don't support the idea and want to assert some snarkiness in there - then best not to attend? They may look bad to you for having a gender reveal, but how bad does it look to show up to an expectant couple's party with nothing?
I don't know what the protocol is, but I would most likely not attend. If I were to attend though I would probably bring something for the parents since the baby shower is for baby gifts. I may bring something like a picture frame or a nice candle or some chocolates. If they were doing a shower/GR combo, then I would just look at it as a shower and bring a baby gift.
I don't think attending without a gift is a way to say GR are stupid. I think these parties are relatively new, so many people probably don't know the protocol. I think it's fine to show up with no gift since the point of it is to gather with loved ones during a special moment.