Are shallots less gas-producing than yellow onion?
This is pretty subjective but I find shallots are right up there with onions (particularly red onions). Of course, not all shallots are created equal, e.g. banana shallots tend to be stronger in both flavour and 'gaseous effect'.

(If you haven't already) I'd suggest trying:


Leeks
Scapes
Wild garlic (if you are in an area where it can be foraged)
Even fennel when cooked long enough (it loses the anise flavour) has been known as a decent substitute