GFORM merge tag change date display format wen using {all_fields}.

GFORM merge tag change date

// Code placement: current theme functions.php

// Change date displayed format.
// Example adding a date field in a dropdown/datefield format is dd-mm-yy and value `09-09-2017`, 
// By adding this filter it will display as `09-September-2017`.

// Take note that in gforms docs: https://www.gravityhelp.com/documentation/article/gform_merge_tag_filter/#5-date-field
// priority is `10` and is working only for datepicker but not for dropdown and datefield, 
// here is changed to `15` to ensure filter would not be overriden.

add_filter( 'gform_merge_tag_filter', 'date_display_custom', 15, 5 );
function date_display_custom ( $value, $merge_tag, $options, $field, $raw_value ) {
    if ( $field->type == 'date') {
        return date_i18n( 'd-F-Y ', strtotime( $raw_value ) );
    }
    return $value;
}

REFERENCE: https://www.gravityhelp.com/documentation/article/gform_merge_tag_filter/#5-date-field