Hello Nicholas,
There is GET.
callback url: http://test.nu/index.php?option=com_cgp&task=callback
Example:
<form method="post" name="cardgate_callback" action="https://menergy.nu
/index.php?option=com_cgp&task=callback">
<input type="text" name="transactionid" value="2194879"><br>
<input type="text" name="transaction_id" value="2194879"><br>
<input type="text" name="site_id" value="3186"><br>
<input type="text" name="is_test" value="1"><br>
<input type="text" name="ref" value="a532058"><br>
<input type="text" name="extra" value=""><br>
<input type="text" name="status" value="200"><br>
<input type="text" name="currency" value="EUR"><br>
<input type="text" name="amount" value="465"><br>
<input type="text" name="billing_option" value="creditcard"><br>
<input type="text" name="transaction_fee" value="0"><br>
<input type="text" name="transaction_fee_bank" value="0"><br>
<input type="text" name="transaction_fee_cgp" value="0"><br>
<input type="text" name="customer_firstname" value="Richard"><br>
<input type="text" name="customer_lastname" value="Schoots"><br>
<input type="text" name="customer_address" value="Maaskade 1"><br>
<input type="text" name="customer_city" value="Oss"><br>
<input type="text" name="customer_state" value="Sa"><br>
<input type="text" name="customer_zipcode" value="5344 KD"><br>
<input type="text" name="customer_countrycode" value="NL"><br>
<input type="text" name="customer_phonenumber" value="2222222222"><br>
<input type="text" name="customer_email" value="
[email protected]"><br>
<input type="text" name="customer_ip_address" value="62.212.71.77"><br>
<input type="text" name="card_type" value="visa"><br>
<input type="text" name="cardnumber_masked" value="111111*****1111"><br>
<input type="text" name="card_expirydate" value="11/12"><br>
<input type="text" name="threed_secure" value="U"><br>
<input type="text" name="hash" value="ef9cd5d38aa72c335724f813e4f7fe23"><br>
<input type="text" name="hashSHA1" value="4c4771e5e74749190e224a297daf48e100acc7ad"><br>
<input type="submit" name="invoeren" value="Invoeren">
</form>
Regards Michel