Yes you are totally right, guro is erotic gore, but it is not that easy to tell which is which because any gore cat be erotic depending on context.
It is not necessary to focus on genitals and lots of guro is not related to genitals at all. probably it could be even said that guro is not as much about gore as it is about absurdity.

However word fetish is also by definition sexually related, so if you are "asexual" you cannot have any fetishes. or you are not asexual after all.