You can't because:>1) I want to reverse engineer Pokemon Sword or Shield
You're not competent enough, especially if you plan on achieving this endeavor all alone.
You probably don't even understand what's in the latest leak, and that it's missing critical pieces of the toolchain, tools and resources that makes it impossible to compile.
2) I want to compile with GNU GCC
You can't build it with GCC alone, the whole engine uses build scripts written in ruby, perl, python, they have resources files in their own proprietary formats and excel files. Also their compiler is probably a modified GCC.
3) I want to play it on my Linux system'd system.
Good luck, dumbest blackest gorilla nigger I've ever seen.