It sounds like you're having a rather odd conversation with this rep. Normally you would need do no more than install a payment module and then configure it in the Admin with data such as account id, password, encryption key, etc. and also configure your account at the payment gateway to give it details of your site such as URLs so that it can ensure that it communicates properly with your site and only your site.
The payment gateway determines what interface approach and format will be used for that communication (in this case it is sounding like XML) and the Zen Cart payment mod prepares and receives data in that format. There is no need for the payment gateway to even know what programming language Zen Cart is using to interface with it. It cares only that the files and messages sent to it are correctly formed, contain all the information needed and are sent securely.
However, payment gateways usually have sample code that can be used to create payment mods (or be built straight into static sites). I suspect that this may be the reason that your rep is asking for this information. But you don't need sample (unless you want to write another payment mod for Zen Cart).
Also, unless you are misquoting him, your rep does not sound like he comes from the technical area as he is mixing file formats (XML) and programming languages (Java and C++) in his question.
To avoid further confusion, there is no Java in Zen Cart. I think you are confusing Java with javascript, which Zen Cart does use, but would not normally be of any significance to a payment gateway.