Hello, Dee. I think it would be considerate and heart-felt to send a late, modest, gift while simply explaining you were not aware of the current day gift giving expectations at a baptism. It's honest and charitable and far from rude.
However, if you choose not to do so, I also think that is an appropriate response. The expectations of some people that "invitation = gift" is rude and greedy and should not be rewarded.
It's really a complicated situation if it involves a decision of appeasing the parents or your own sensibilities in sharing the awe and wonder of the baptism of a child.
What it all boils down to is your intentions. I think you will find the best solution in your heart: A baptismal, material, gift is not equal to a lifetime commitment of watching over that child with your love and compassion.
Your eager witness to this sacred ceremony will stand alone and far above any material gift. May God bless you for this act of faith, alone, material gift or no material gift!