There's nothing obnoxious about baby gender cake parties. The event, whether it be a baby shower or whatnot is already centered around the parents as it is. It's a fun idea and if the guests are offended, they don't have to come.
I did a gender reveal party just a few months back, and I think its an exciting and fun event to have. I did not expect gifts at all, just the presence of those who we are close with to share in finding out the news. Its supposed to be fun, not a burden. We had appetizers and of course, the cake, and all we wanted was for people to share in the joyous news with us!
I like gender parties, once its happening the same time as the baby shower so that I won't have to buy 2 gifts, nor 2 outfits nor 2 pairs of shoes :D