The only way the bootloader can be changed is with a programmer like the pickit. It should always be possible to reinstall the firmware using an SD card.
I’m not sure why some gateways are unable to boot after erasing the serial flash. The serial flash does store the OTA firmware updates downloaded from our update server, so my suspicion is that the gateway thinks that all those zeros in the serial flash (after erasure) are the newly downloaded firmware and tries to run it. I’ll forward this to the gateway team. Maybe they can explain what happens.