I installed the firmware version dated 19/1/2020 on the GD77 and on the RD-5R. I measured the UHF output level on a dummy load with a diamond SX-600 in PEP mode. There is not other difference between the two measurements other than the GD-77 needs a SMA gender changer, as the antenna plug is different between the two devices. So: everything identical up to the antenna plug.
These are the results:
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Ref level 0,5 1 2 3 4 5
GD77 FM 0,3 0,5 1,1 2 2,9 3,9
RD5R FM 0,05 0,5 1,1 1,9 2,5 3,6
GD77 DMR 0,3 0,5 1 1,75 2,3 3
RD5R DMR 0 0,1 0,5 1 1,1 1,5
I am not particularly worried about the FM levels being smaller than they should be. There are losses in the cable, I did not calibrate the meter, the battery was not 100 full (about 70% on both), etc... I am not particularly worried about the GD77 DMR levels being a bit different than the GD77 FM levels, the power meter is not designed for digital signals, even if it does not appear to work too badly. I am also not very worried about very small levels being all over the place, as you said the firmware can only do so much.
But I feel concerned that the RD5R DMR levels are half of the GD77 DMR levels. How can I help you addressing this problem?