Nix or Guix apps install themselves in stores. A app can have dependencies linked to another store - it will/should never use the libraries available by the system package.
Normally when I execute a simple GTK app installed through Nix, inside a Nix shell for example, it should use the GIO library from the store and not the system library. When the later happens (due to some bug/faulty code), it is called a environment contamination.
In this scenario, the app may or may not work - if the versions are very close-by, it should work just fine. However , if you’re using an older channel on a bleeding-edge distro like Arch, or a beta channel on a slow distro like Linux Mint, you’ll definitely have a hard time.