J.-M Friedt has created a block for GNU Radio called gr-rpitx which allows a Raspberry Pi to be used directly as an output RF sink in GNU Radio. If you were unaware, RPiTX is software that allows you to turn your Raspberry Pi into a transmit capable SDR without any additional hardware apart from a wire antenna connected to a GPIO pin. It works by modulating a GPIO pin in a way to generate any arbitrary signal modulation. gr-rpitx allows this software to be used directly within GNU Radio.
In his presentation uploaded early for the upcoming online European GNU Radio Days conference, J.-M Friedt explains how gr-rpitx works, and shows how you can easily connect any flowgraph to the gr-rpitx output sink. His examples demonstrate retransmitting broadcast FM using an RTL-SDR, broadcasting digital signals like DRM, and how gr-rpitx and RTL-SDR could be used as part of a basic scalar network analyzer.
gr-rpitx uses the GPIO4 output of the Raspberry Pi to generate a radiofrequency stream fed by a GNU Radio signal processing flowchart with sample rates up to 400 kS/s.