Salesforce API Gravity

I have found the “fix” to the Lead Assignment rules not being triggered in Salesforce but I am not clear where the “fix” should be implemented.

It simply reads:

add_action('gf_salesforce_connection', 'gf_salesforce_set_default_assignement_rule');

function gf_salesforce_set_default_assignement_rule($client) {
    //  Two Options for Setting Assignment Rule
    //    1.  Pass in AssignmentRule ID and "false" to use a specific assignment rule.
    //    2.  Pass in null and true to trigger the DEFAULT AssignementRule
    $header = new \AssignmentRuleHeader(null, true);

    $client->setAssignmentRuleHeader($header);

    return $client;
}

Does this exist somewhere or does this need adding somewhere to the php?