arm_save_payment_gateway_settings

add_filter( 'arm_save_payment_gateway_settings', 'your_function', 10, 2);
function your_function($pg_settings, $posted_data) {
    //$pg_settings Payment Gateway Settings Array.
    //$posted_data Payment gateway form posted data.
    $pg_settings['paypal']['some_key'] = 'jr738ds4dj8fh5dk27djdn';
    return $pg_settings;
}