Well, you can just use the interfaces, and create a bespoke provider for the interface. There is no real need to create a bespoke gateway provider.
But if you are only ever going to use 1 payment gateway then I think I would take the approach of using the existing Authorize.Net code and just add the code to create a http post in there. I think it'll be quicker to write and test, that's all.
If in future you might change gateways or have multiple gateways then I think you need to create a bespoke interface provider which use those interface classes I've listed. Although I don't have any example code to start you off. So if you do go down that road please think about donating a skeleton code which other people can use.
But if you are only ever going to use 1 payment gateway then I think I would take the approach of using the existing Authorize.Net code and just add the code to create a http post in there. I think it'll be quicker to write and test, that's all.
If in future you might change gateways or have multiple gateways then I think you need to create a bespoke interface provider which use those interface classes I've listed. Although I don't have any example code to start you off. So if you do go down that road please think about donating a skeleton code which other people can use.