It's a gadget. And it is GUD.
GUD is an open USB display protocol. The host driver has been available in mainline Linux kernels since 5.13. The host sends display output to a USB device.
An official gadget implementation exists as a Linux kernel module, but it has not been mainlined, and (as of writing) does not build on the latest 6.x kernel releases.
The gud-function crate implements a GUD gadget as a FunctionFS function, for use with the usb-gadget crate.
The gud-drm crate is a simple implementation that configures a GUD gadget with the gud-function implementation, and renders the pixel data directly to a drm framebuffer.