Abi, I think it depends on the age of the child and the tastes of the parents. Dd1 was baptised in a longish gown, actually a victorian baby's petticoat! because I found our family's proper baptismal gown too elaborate. Also she was 8 months and even the petticoat was a bit tight on her.
Ds was 14 months, so slightly beyond the age for gowns. He wore a little white linen sailor suit that a friend bought for him in France, and v. cute he looked too.
Dd2 was slightly younger, and I actually bought a dress for her, cos she's my last baby, we had a big party afterwards and I wanted to indulge myself by having her looking special for it. All were baptised in v. trad Catholic church, btw.
Moondog, I don't think there's anything wrong with wanting to dress your child up for a particular occasion. I don't think you should judge someone's motives on their choice of clothing for their baby or themselves. Dressing up is one of the ways in which we mark the specialness of an important occasion, whether a wedding, a baptism, First Communion or graduation. Presumably you don't show up to weddings in your old jeans?
Abi, I hope the day is wonderful for you and your little one.