Oh my bad. Yeah you can configure almost anything in a declarative way on the /etc/nixos/configuration.nix file:

services.xserver.enable = true;
services.xserver.displayManager.gdm.enable = true;

Or

services.xserver = {
  enable = true;
  display manager.gdm.enable = true;
};

You can define a lot more (programs, users) and then you build the config file to take effect. By default NixOS is immutable so you can't change anything in the root partition and if there is something that broke you can rollback to a previous build!