Many people fail to set these (and other) flags on profiles, I don't think it's necessarily an issue with Gateway Ancestors only. The trouble is that it's a multi-step process. You first have to create a profile, then set the marriage flags on a different page, then set flags for confidence & whether they have no more siblings and children, then if they died young you also need to go to yet another page to flag them as unmarried. It's tedious and far from intuitive.
Most just can't be bothered.