What does "you can't gaffa the gaffa" mean?

Gaffa is British slang for "boss".

The phrase you saw means "you can't boss the boss"

Note that boss can be used as a noun

Your boss is the person in charge of the organization or department where you work.

and a verb

If you say that someone bosses you, you mean that they keep telling you what to do in a way that is irritating.

The spelling of "gaffa" derives from gaffer

People use gaffer to refer to the person in charge of the workers at a place of work such as a factory.

This is an informal use of the word.

Formally a gaffer is the senior electrician responsible for lighting in films or TV productions. The gaffer's assistant is known as the best boy.