IMHO:
Couples who wish to marry are not obligated to throw an insanely expensive party to entertain their family and friends.
An invitation to a wedding reception is not compensation for a wedding gift.
The kind, size, or location of a wedding ceremony - or whether I'm invited - has nothing whatsoever to do with whether or not I'd give the couple a gift. If they're my friends or my family and I want to wish them well, I'll give them a gift.
It's just like any other gifting occasion. I don't need to be invited to a birthday party to give someone a birthday gift, and I don't need to be present at the birth of a baby to give someone a baby gift. Why would I need to be present at the wedding to give a wedding gift?