A function with filter for wp to enable the use of merge tags in gravity forms fields before form submission

functions.php

<?php
/**
* Merge Tags as Dynamic Population Parameters
* http://gravitywiz.com/dynamic-products-via-post-meta/
*/
add_filter('gform_pre_render', 'gw_prepopluate_merge_tags');
function gw_prepopluate_merge_tags($form) {
    
    $filter_names = array();
    
    foreach($form['fields'] as &$field) {
        
        if(!rgar($field, 'allowsPrepopulate'))
            continue;
        
        // complex fields store inputName in the "name" property of the inputs array
        if(is_array(rgar($field, 'inputs')) && $field['type'] != 'checkbox') {
            foreach($field['inputs'] as $input) {
                if(rgar($input, 'name'))
                    $filter_names[] = array('type' => $field['type'], 'name' => rgar($input, 'name'));
            }
        } else {
            $filter_names[] = array('type' => $field['type'], 'name' => rgar($field, 'inputName'));
        }
        
    }
    
    foreach($filter_names as $filter_name) {
        
        $filtered_name = GFCommon::replace_variables_prepopulate($filter_name['name']);
        
        if($filter_name['name'] == $filtered_name)
            continue;
        
        add_filter("gform_field_value_{$filter_name['name']}", create_function("", "return '$filtered_name';"));
    }
    
    return $form;
}

You should add this to your child theme's functions.php, not replace it or add it to the base theme because it will be overwritten on update.