Since I last posted on this subject, I've found an... eclectic three-movie collection, featuring two of the early '90s-era Godzilla movies and one from '01. One of these movies features a confusing time travel plot but is probably supposed to be in continuity with the other one. (It also features this scene.)
The other one is (as per the common rebooting policy of Toho film continuity) only in continuity with the original Godzilla movie, and features Godzilla coming back as a sorta-zombie that apparently wants to avenge the dead of World War II. ...Yes, really.
And one where he fights Mothra. (Well, two, because the ghosty-zombie-whatever one also features him fighting Mothra.)
So, in terms of "releases taking advantage of Godzilla movie," all they'd need is Godzilla: The Series to tie things up at this point. (Pricewise, Gamera's definitely the winner, because we're talking about twelve dollars versus a large number of ten-dollar-plus releases.)