Gatekeeping means controlling or filtering access to a valued institutional opportunity.

For example:

Anyone can write a paper.

But:

Not every paper is published in a prestigious journal.

Anyone can write a book, but:

Not every book is adopted into university courses.

Anyone can present an interpretation, but:

Not every interpretation receives conference invitations or scholarly attention.

So freedom of expression can coexist with unequal distribution of attention, legitimacy and institutional opportunity.