Is what the idea of SPI? SPI just happens to be a protocol that is commonly used in integrating two different microprocessors together.
The concept of Gadgeteer is to provide easy ways for people to connect common building blocks in a usable prototype or small production runs. It’s not a be-all-end-all list of every single device (like your Microchip digital potentiometer), but it gives an extensible way to make these things accessible to people should the need arise. It is designed to allow simple ways to interact with these devices - so that you don’t need to know the fundamental SPI communication layer for instance, or the specific SPI commands to interact with the processor - you just need to use the high level functions that the module driver exposes.
For example, high level functions for a digital pot might be
Below that, the code would need to know how to set a particular resistance via the SPI commands that need to be issued to the chip. Below that, the driver needs to know how to talk to the device over SPI. Below that, the netmf implementation needs to know how to talk to the SPI peripheral on the mainboard processor (and you don’t need to worry about that as either an end-user or a module designer).
So in your case, because you’re setting out on this journey, you first need a way to connect your physical digital pot to a Gadgeteer mainboard - that’s the hardware part, where you will wire things up. As I said, a breakout module and a MCP device on a breakout board is probably the simplest way to achieve that.
You then need a way to communicate to the MCP device - there’s code for that already, but you can create your own if desired, or extend it for your needs. If you only need to use the device and you aren’t intending to develop this more fully as a module, then you really don’t need to do much more than what the existing code does - you certainly don’t need to “Gadgeteer-ise” it