> How is unionization gatekeeping?

If you are in a right-to-work state and you don't join the union, then union members know you're benefiting from the union without contributing back. Historically, this leads to an uncomfortable work situation for you.

If you are not in a right-to-work state, and the collectively-bargained contract involves a union membership requirement - which is typical? - then you have to join the union if you want the job.

This is where the gatekeeping concern comes from.

> I can't see any disadvantage for the employees in joining a union.

Unions have dues, so you're giving up part of your salary for membership in the union. If the union salary is equal or less than you would be able to negotiate on your own, it's a disadvantage for you.