@Thomlodz
Rather than use command line, load the RGN file by dragging it onto Updater.exe so the GUI opens then try connecting the edge 800 in preboot. The device details should appear in the GUI window and move very quickly to click 'OK' or just hit Enter key on PC keyboard.
PS: Use safe naming rather than gupdate.rgn because *.rgn will force a flash even if the HWIDs of the firmware and device don't match which can hard-brick and likely kill the device. Safe naming format is ONLY XXXX01000xxx.rgn where 'XXXX' is the utilized FIRMWARE'S HWID and 'xxx' is the fw version without decimal point. The '01000' tells Updater.exe to first check that the RGN file name's HWID matches the Device's HWID eliminating the chance of unintended flashing of a foreign boot.bin. Even changing '01000' will force flashing like any naming other than the safe format. GarminCure3.exe will use that safe naming automatically. If using QuickCure3.RGN rename it manually although it may not work with edge, i've not tried it on any outdoor device.