I’m trying to understand, since it’s DRM-free, can I buy it and share with many people so they can play it on their PC?
You are not supposed to
Or there’s something that prevent me to do that?
Nothing technically
Does it mean that pirating GOG games is easy as transferring to other machine?
Yes
If so, I wonder why pirate sites like FitGirl still use cracked Steam version of games, why don’t just share GOG version?
Repacks are sourced from the scene, and the scene cracks video games. The fact that you get free video games out of it is a side effect.

Nicks and Russel Mocs both have models with zippers and they are about as quality as they come.