It's not a "flag", really: it's a fundamental, implicit underlying assumption about the execution environment. The C extensions assume (roughly) that if they're executing, they can touch any part of the runtime without any synchronization whatsoever, because nobody else is executing (that's what GIL ensures).

Or, to put it another way, it's the first letter of the acronym: "Global Interpreter Lock".

You can't really "fake" a global lock. Either you're globally locking everywhere with the global lock or you aren't.