Oh it's even weirder than that: whether or not paths are separated by colons or slashes is specific to the app you're using, even in modern macOS versions.

To see this in action, open a terminal, type:

  touch test:file.txt

Then open Finder and browse to whatever folder you were in and you'll see a file called test/file.txt.

I'm not sure which is the native representation, but I do know that every API I've used to interact with the filesystem on macOS has used slashes as the path separator, so GP should be fine. But it's definitely a fun party trick to pull out around unsuspecting Mac users.