add_filter( 'arm_membership_setup_gateway_option', 'your_function', 10, 3);
function your_function($gateway_fields, $gateway_key, $gateway_options) {
    //$gateway_fields Payment Gateway fields html.
    //$gateway_key Payment Gateway Key.
    //$gateway_options Payment Gateway Settings Array.
    if($gateway_key == 'custom_gateway') {
        $gateway_fields = "Your Gateway Fields Html.";
    }
    return $gateway_fields;
}