There doesn’t appear to be any concrete information stating that Gordon Freeman indeed is autistic. Does he display attributes that scream autism? Sure, but does that mean he actually is? Like some autistic people, Freeman excels in science, he doesn’t speak and he is not emotionally invested in others. Frankly, he appears to be aware of his surrounding and couldn’t it be possible that he chooses not to speak so that people will find him to be peculiar?