GTA 5 hasn’t been licensed to you and you are absolutely not entitled to read it, even if you managed to get hold of it due to a theft. By reading it as an app developer you taint your knowledge with stolen intellectual property and stolen trade secrets, potentially exposing yourself and any game you work on (including for an employer) to criminal and civil penalties.

That’s the immense value of open source and Linux in specific. You are allowed to read it, improve it, rip out bits that are useful (as compliant with the license), and use the concepts as fully licensed intellectual property without trade secret encumbrance.

I am personally really interested in reading the source and see how they do things. I’m certain there’s fascinating bits of tech in there. But I wouldn’t underestimate the risk I would put myself, my family, and my employer at and the willingness of corporations to crush the small guy. See the pain inflicted by downloading mp3s, and the marginal value of copying an mp3 is infinitesimal compared to the source code of a AAA game to the studio.