urlencode query string in gravity forms confirmation redirect

After digging a little deeper, I found that there is an action hook in Gravity Forms called gform_pre_submission. This allows me to modify a posted value before creating an entry or running the confirmation (see documentation).

After that it was pretty basic:

add_action( 'gform_pre_submission_11', 'pre_submission_handler' );
function pre_submission_handler( $form ) {
    $_POST['input_7'] = urlencode(rgpost( 'input_7' ));
}

This targets form ID 11 and replaces field 7 with an urlencoded version of field 7.

Job done 🙂