HDR BT.2020 to SDR BT.2020 Absolutely Does require Gamma 2.4 when used with a UB900/HDFury/JVC combo
If you set the Panasonic to convert HDR to SDR it will revert down to SDR Rec.709 8bit (same as bluray) Don't do that!
What many JVC users have been doing (since Summer 2016) is using an HDFury Integral/Linker/Vertex to change the EDID info in the signal
to allow the Panasonic to send SDR BT.2020 with Wide Golor Gammut because the HDFury product turns off the Flag for HDR but leaves everything else intact.
The other benefit was it restored the use of the Dynamic Iris that is not functional with HDR in the signal.
The EDID on the Vertex for SDR BT.2020 WCG number 14 (if your HDMI cable is 4K 4:4:4 12Bit capable)
Yes you would use BT.2020 and Gamma at 2.4 for that (all your settings would be very close to your 1080p settings)
Panasonic re-maps the colors perfectly for SDR BT.2020 conversion, something Oppo has been struggling to match since its 203 release.
When Custom Gamma Curves and Arve's tool came out about last Spring, many switched back to using HDR.
The Linker or Vertex would keep the JVC in your new custom gamma selection, otherwise it will auto switch back to JVC's broken Gamma D.
If you want to use a Custom Gamma you select BT.2020 and load up to 3 different custom curves into the JVC's memory slots.
I bought the Vertex because my Integral could not stop the gamma D auto switch, or enable the DI in HDR mode.
Going to try the Oppo's new FW update as soon as it gets an official release,
I can't load a Beta FW because my 203 has the region free mod