Children referred to the UK’s largest gender clinic were vastly more likely than average to present with autistic traits.
One study  noted that:
48% of children and young people who were seen in GIDS and whose parents completed the social responsiveness scale (SRS), a quantitative measure of autistic behaviours in children and young people, scored in the mild to severe range.
A BMJ paper  reported:
Around 35% of referred young people [i.e. referred to the GIDS] present with moderate to severe autistic traits.
 Churcher Clarke, A. & Spiliadis, A. (2019). ‘Taking the lid off the box’: The value of extended clinical assessment for adolescents presenting with gender identity difficulties. Clin Child Psychol Psychiatry 24 (2): 338-352. [Link]
 Butler, G., De Graaf, N., Wren, B. & Carmichael, P. (2018) Assessment and support of children and adolescents with gender dysphoria. Archives of Disease in Childhood103:631-636. [Link]