Laravel.io
<?php

namespace App\Providers;

use Laravel\Horizon\Horizon;
use Illuminate\Support\Facades\Gate;
use Laravel\Horizon\HorizonApplicationServiceProvider;

class HorizonServiceProvider extends HorizonApplicationServiceProvider
{
    /**
     * Register the Horizon gate.
     *
     * This gate determines who can access Horizon in non-local environments.
     *
     * @return void
     */
    protected function gate()
    {
        Gate::define('viewHorizon', function ($user) {
            return ends_with($user->email, '@signifly.com');
        });
    }
}

Please note that all pasted data is publicly available.