app/code/Gta/AdminPay/Plugin/Payment/Method/Checkmo/Available.php

<?php
namespace Gta\AdminPay\Plugin\Payment\Method\Checkmo;

use Magento\Customer\Model\Session as CustomerSession;
use Magento\Backend\Model\Auth\Session as BackendSession;
use Magento\OfflinePayments\Model\Checkmo;

class Available
{
    protected $customerSession;
    protected $backendSession;

    public function __construct(
        CustomerSession $customerSession,
        backendSession $backendSession
    ) {
        $this->customerSession = $customerSession;
        $this->backendSession = $backendSession;
    }

    public function afterIsAvailable(Checkmo $subject, $result)
    {
        if($this->backendSession->isLoggedIn()){
            return $result;
        } else {
            return false;
        }
    }
}