Create your own UCM Payment Gateway plugin
This requires moderate to advanced levels of PHP knowledge. And it's probably a good idea to only try this if you have implemented this payment gateway before in another system (ie: so you know how the payment gateway code works).
The easiest way is to just copy the PayPal plugin ( includes/plugin_paymethod_paypal/ ) over to your own payment gateway folder (eg: includes/plugin_paymethod_mygateway ). Then do a find replace from "paypal" to "mygateway" in all the file names and file contents.
Once that is done pop open the new "mygateway.php" file and modify all the PayPal logic so it will talk to your gateway.
Send us a support ticket if you really get stuck.