Is Gibson an admitted sedevacantist? I take it that the Tridentine Roman Missal may still be practiced, and is even welcomed by the today's church leaders. I had understood that one of Vatican II's reforms in allowing the missal to be conducted in the primary language of a congregation was simply to make worship more accessible for those who preferred the vernacular.
Apparently some sedevacantists, possibly Mel's father Hutton, believe there has been no pope since the conclave that elected John XXIII was "subverted by a Jewish/Masonic conspiracy." This quote describing Hutton's attitudes is taken from a critical article in Catholic Citizens of Illinois called Tribune Media Turns to Dissenters to Mislead Catholics: Sneering Through Gibson's PASSION.