This template defines the properties required to deploy a VirtualNetwork Gateway in Azure.
It appeares that the ApiVersion above can't actually be found the Microsoft.Network.json appears to be missing from the folder. I've created an issue for this here.
This resource doesn't have a defined properties object, everything is referenced as properties.bgpsettings, this is not an issue just odd, considering the last few I've worked through had a properties object of some sort.
It seems we have a stray type, this is outside of the properties and I don't see it referenced in the examples or the schema
The property values for this property are not documented outside of the schema. They are documented in the arm template documentation.