It's not a wedding. It's a BBQ celebrating a wedding that happened three months ago. If they wanted presents, they'd have invited the OP to the wedding, or sent out announcements immediately after, letting everyone know that they'd been married.
The invite to this event just sounds way too informal, fun, family-ish and silly -- it's a summer BBQ party so that friends of the couple can gather, celebrate, eat, drink and be merry. To me ... that doesn't automatically say, "Buy me stuff." YMMV