This assertion seems to be implicit or explicit in every argument by trans-inclusionists where the distinction matters.
Preferred pronouns are obviously an assertion of the primacy of gender.
The accesssion of transwomen to positions representing women, including in matters of diversity hiring, is another obvious assertion of the primacy of gender.
Then of course there's the big one we've been debating here: Access to sex-segregated spaces. In restrooms, locker rooms, women's shelters, and women's sports, there is the assertion that gender is more important than sex.
Then there's Boudicca's rather extreme (but certainly not unique) assertion that the biological underpinnings of her gender identity disorder are actually what define her sex.
About the only context in which the primacy of gender over sex has not been asserted has been in the bedroom. And even there, I think that some are already asserting it.
So yeah, I can't think of any context of importance where trans-inclusionists don't assert that gender is more important than sex.
ETA: Ah! Medical care. And even there, trans-inclusionists push for as much gender-oriented care as possible.