Excuse me if this has already been answered, but I would appreciate a list of examples of this condition for the purposes of loading up the Dep Ones Rising Track. My main question is if there is already a gate on a location indicated by a mythos card--a monster surge situation, usually--does this count as "preventing" a gate from opening, and thereby worthy of an uprising token?

A Gate opening prevented usually means:

a) The location has an Elder Sign, or

b) Kate is standing in said location.

A monster surge is not a gate opening, but happens instead of a gate opening, so it does not count.