I never heard of R’ Ralbag saying regular gelatin is kosher. I do know that he allows a synthetic gelatin to be used. Even if he does allow it, I never heard of any other Rav allowing it. In fact, every MO Rav I’ve ever spoken to told me not to use the Triangle K hechsher.
As for the UWS eruv, I honestly don’t know much about it. I assume there is a Rav they rely on. But to your point saying that nobody can argue on R’ Moshe zt”l, would you say the same about chassidim who use the eruv in Brooklyn? After all, R’ Moshe made it very clear that an eruv couldn’t be made in Brooklyn either. Or do you think chassidim can argue on R’ Moshe, but MO can’t?
My Rav told me he follows R’ Moshe with the eruvin. Most MO Rabbonim I know do the same. I did speak to a Rav who was a talmud of R’ Tuvia Goldstein zt”l, who told me R’ Tuvia held the Brooklyn eruv was allowed, and could be relied on. Would you say R’ Tuvia was able to argue on R’ Moshe?