will have had supported adult transitioners and detransitioners
Again, to be precise, I'm not sure he worked directly in gender-related treatment. Tavistock is a broader psychiatric treatment centre, of which GIDS is only a (rapidly growing!) part.
But it's clear that he's able to apply his general psychiatric experience to their overall approach.
Much of the problem with GIDS appears to have been obsession over "gender" while ignoring (or forgetting) everything else we know about people, and the sort of issues they may have.
GIDS are saying "these are trans kids", and he's saying "no, these are kids with a whole host of issues, often including autism or internalised homophobia, or homophobic parents, and they should all be looked at".