Zorro, glad our comments helped you feel better.
Daisyboots, yes, I think I would feel the same way - at first. But when you think about it, maybe this wasn't based on a decision he made alone. Perhaps it reflects something he and his bride decided together - for example, "only blood gps and their spouses invited to the ceremony plus reception, steps only to the reception." It sounds cold, even as I write it, but if his bride has any steps, it might be based on how she feels, and they may not think it's fair to treat his steps differently.
It's also possible that one of the blood gps has voiced an objection to sgps being at the ceremony, as ridiculous as I think that is. Your sgs and his bride may have been pressured into pleasing that awful gp. Or, sad to say, some foolish person may have objected to steps being present, at all, and this was a compromise.
Also, so often, Iv noticed people plan weddings and guest lists according to what they think is "supposed to" be, instead of how people feel about each other. I don't mean everyone, I know some people who go strictly by what they feel about each prospective guest. But Iv also known some who go by what they think is "correct."
Or it may be as you say. Anyhow, so sorry you've been hurt by this. You don't have to go, of course, and if not, you certainly don't have to send a gift. However, I would err on the side of being generous, give sgs the benefit of the doubt and sens a little something.