How is unionization gatekeeping? I honestly don't understand what you mean. I can't see any disadvantage for the employees in joining a union.

To be fair, at this point in my life I think unions are a net positive and probably the most effective protection workers have from predatory management, but the 25 year old libertarian take on this is issue is based on things like unions lobbying states to require licensure. Restricting entry obviously benefits incumbents, which is the very definition of gatekeeping, and it would have specifically hampered a self-taught engineer like myself.

There are enough cases of unions protecting bad actors (cops, prison guards) or lazy, tenured individuals that it's easy for a mildly privileged autodidact to decide they don't need the hurdle - or the help.

The basic principle is everyone gets the same pay, meaning if you are someone who wants to put in a lot of work to rise up quickly, it won't happen.