The GNU software is insignificant compared to the rest of what is included in the operating system. Most casual users won't even actively use GNU software.
You still need it. I'm struggling to imagine any sense in which GNU is less vital to the functioning of the system than Linux, though it's true that both are potentially somewhat under the hood components.
What use is it? Shell scripts are no longer relevant now that distros have been modernized by systemd. Sure there may be some legacy things hiding somewhere that depend on GNU bash or GNU coreutils, but those are usually not user visible. ait is only vital in the sense of backwards compatibility.
GNU includes glibc, which is used by most binaries in the system, including systemd and user applications. Beyond that, I think you underestimate the degree to which coreutils is used. For fun, why don't you try uninstalling them? You might be able to remove bash, but I would be shocked if you can boot without coreutils.
