Authorize.net Return URL

[Guest maolaun]

This is my first time working on a shopping cart module and I am also fairly new to php, so please forgive my lack of knowledge.

I am currently using the Authorize.net module (focusing on the SIM method) as a guide to build a new module for a different payment gateway. So far I have been able to submit the POST to the gateway correctly, however, I am running into an issue with the return url. The gateway currently scrubs any ampersands or equal signs (&, =), so using the default authorize.net return url does not work:

$GLOBALS['storeURL'].'/index.php?_g=remote&type=gateway&cmd=process&module=Authorize

is converted to

$GLOBALS['storeURL'].'/index.php?_gremotetypgateway&cmdprocessmoduleAuthorize //with a bunch of GET variables tacked on the end.

Therefore, when I am redirected from the hosted payment page back to Cubecart, I receive the "Page not found" error. Could someone please explain what the following section of the return url is accomplishing: ?_g=remote&type=gateway&cmd=process&module=Authorize

I could have the return url point to a new file that I could create (to eliminate the & and = character issue), but I want to understand how the above url is interacting with the cart first.

Any help is greatly appreciated.