Hi rlkeck, unfortunately we have seen a number of fake RSP devices appear on the market and so now we don’t publish circuit or even block diagrams of the hardware. The published API shows the way the gain stages are set. See https://www.sdrplay.com/docs/SDRplay_API_Specification_v3.06.pdf
there is progress now on getting updated GNU radio source blocks.
Franco Venturi is a key developer of GRC source blocks and author of https://github.com/fventuri/gr-sdrplay3
Best regards, Jon