Gravity Forms PayPal Standard Add-On

GFORM: Change Currency

/**
 * Change payment currency base from another field, when specific form is submitted.
 */
add_action( 'gform_pre_submission_2', 'change_currency_' );
function change_currency_( $form ) {
    add_filter( 'gform_currency', function() { return rgpost('input_4'); } );
}