Allowed_Email.js

jQuery(document).ready(function($) {
    $('#registerform').find(':submit').click(function(e) {
        if( ! isAllowedEmail( $('#user_email').val() ) ) {
            e.preventDefault(); // Prevent the form from submitting
            alert('You can only register with an @gmail.com email address!');
            return false;
        }
    });
});

function isAllowedEmail( address ) {
    return address.indexOf("@gmail.com", this.length - 10) !== -1;
}