Gillick competence remains but it seems to have a very low bar.
Some of the stories in Hannah Barnes' book Time to Think suggest that the children involved had no real understanding of what they were saying yes to. Also the co-morbiditities were effectively seen as irrelevant.
I'm very glad that my daughter told the CAMHS therapist that she wanted to remove her lower right leg. However, in a year's time, in theory, my daughter could say she realised how ridiculous that was and this in itself could be considered proof that she had reached an age of Gillick competence, even when she remains incredibly vulnerable owing to her low cognitive processing skills (which are masked by her wide, intelligent-sounding vocabulary). There will be many autistic children who have a vulnerability similar to this.