arm_change_payment_gateway_tooltip

add_filter( 'arm_change_payment_gateway_tooltip', 'your_function', 10, 3);
function your_function($titleTooltip, $gateway_key, $gateway_options) {
    //$titleTooltip Tooltip Content.
    //$gateway_key Payment Gateway Key.
    //$gateway_options Payment Gateway Settings Array.
    if($gateway_key == 'custom_gateway') {
        $titleTooltip = 'Configure custom payment gateway from your example.com account.';
    }
    return $titleTooltip;
}