I got two responses from CDPR support team:

(Simplified version of first question/answer)
Me: Why there are differences between GOG and GOG Galaxy releases (+listed different files).
Support: Contact Gog for clarification.

(Simplified version of second question/answer)
Me: OK, but you must know, if \\content\\content4\\streaming.cache has to be present or not?
Support: I'm sorry but I'm not allowed to share all the production details, however this is by design.

So basically the answer is ]\\content\\content4\\streaming.cache is in GOGs and Steams version on purpose and is not present in GOG Galaxys version on purpose too, because this is a design decision.