Gate2Shop
From WHMCS Documentation
Setup
To activate the Gate2Shop module in WHMCS, navigate to Setup > Payments > Payment Gateways and choose Gate2Shop from the Available Gateways dropdown.
Callback URLs
Next login to your Gate2Shop control panel and configure the Callback URL settings as follows:
Success URL
http://www.yourdomain.com/whmcsdir/modules/gateways/callback/gate2shop.php
Pending URL
http://www.yourdomain.com/whmcsdir/modules/gateways/callback/gate2shop.php
Failure URL
http://www.yourdomain.com/whmcsdir/modules/gateways/callback/gate2shop.php
Back Page
http://www.yourdomain.com/whmcsdir/clientarea.php?action=invoices
Where yourdomain.com/whmcsdir/ is the location of your WHMCS installation - adjust this appropriately to point to your actual WHMCS location.