Thank you for your answer, and sorry for the late reply

Ok I think I understood the mechanism, but I don't understand how to use this regular expression (the one given by, for example,
(org-tags-expand "GTD")
which gives me the regular expression "with the other tags"
)

I see perfectly the idea : a search is done with org-agenda with this regular expression. But I can't use it.

The problem is that I don't know how to use this regular expression with a function that returns the tags. Do you have any clue (piece of code where it is already used ? A function allowing me to translate this regular expression into a simple tag? Do I just have to convert this regular expression into a list of tags? Is it possible? I'm asking myself all these questions because I just don't know where to go ^^)

Thanks in advance for your future answer.