Extract XML/JSON element from Zillow API response and populate into Gravity Forms field

the first step is to extract the value with this kind of code, does it work for you ?

$xml = simplexml_load_string($result);
$value = (string) $xml->response->results->result->zestimate->amount;

var_export($value, FALSE);

the next step is to put the value in the form. the Gravity Form is not free then we cannot test it and I only find that in documentation : https://docs.gravityforms.com/gform_get_field_value/
if that doesn’t help, you have to ask to gravity form support.