Most people I know who went to college, but not immediately after high school, went to work or military service after high school. It is not like gap years are a new thing*, though, at least on these forums, “gap year” sometimes has the connotation of a structured program funded by high SES parents more than typical post-high-school activities that are not college.
It is also quite common for students graduating from college but planning to go to graduate or professional school to do something other than school in between – usually working to pay off student loans and/or build up savings, though some types of professional schools (e.g. MBA programs) also favor those with post-college work experience.
Another variant is the gap semester during college in the form of a co-op job.