The latest core snap in stable hasn't yet been validated for the Gateway 5000. It's expected that this will happen first thing tomorrow. As Facundo mentioned, there's a bug that's being worked on which causes the error message when snapd tries to download an older version of a snap that is no longer published in an accessible channel. Once the latest stable core snap is validated for the hardware, the error will no longer happen and the core snap will be refreshed.
If you'd like to install now, you can do so using this command:
$ snap refresh core --ignore-validation