My mother in laws box was stuck with red light on i managed to flash it using 8gig memory stick it is possible just takes time and patience they thought it was no good and I told them I would try and sort it and managed to do it but they didn't have problem with display
And there in lies the difference, the OP states he has no LED display.
The following is assuming NO hardware problems at all, including NO PSU problem.
When the Zgemma is powered up it first runs a preloader routine which among other things, after checking the integrity of EEprom/NVram initiates the LED display, then the USB ports are checked for a flashable Bootloader image, (if found,displays "UPDT" ) and then calls the Bootloader routine (displays "BOOT"), this checks for a firmware/build update (if found, displays "FLSH" ), if not found continues to load the firmware.
Look at it this way, like in the PC world :-
Preloader = CMOS (preloader cannot be flashed but can be Jtagged)
Bootloader = Bios (can be flashed, but is NOT part of the firmware/build flash)
Firmware/Build = OS such as Win 10 etc.
So to sum up, what is generally referred to as a re-flash, within these threads, is the Firmware/Build re-flash and as the bootloader image is not within these flash files it follows that it does not get reflashed/repaired.
If either the preloader or the bootloader become corrupt they must be repaired before a successful Firmware/Build flash.
As I said in my first post, in this thread, a common problem causing red light/no display is a faulty PSU which intermittently may allow a successful firmware flash and appear as if the problem has been rectified but no doubt will raise its ugly head again.
If you think about it logically, a raw machine, with no build on it will sit there displaying "BOOT" ad-infinitum until you flash some firmware on it. If its not even getting that far then the problem is not within the firmware, so a simple re-flash will not fix it, or at least not permanently.
The above is just my understanding of the hardware/firmware setup within zGemma and is based on, what little I may know about them, but also 40years experience within the electronics industry. As always I am open to correction, but please back it up with logical reasoning and not just something like, "Well it worked for me" etc.
Cheers.