Answer
You can load a specific version of firmware by using the following URL with replacement of <ga_version>
https://connectivity-firmware.s3.amazonaws.com/rg1xx-lora-gateway/firmware/<ga_version>
For example, GA6.2 is desired firmware to be downgraded to, the URL should be the following:
https://connectivity-firmware.s3.amazonaws.com/rg1xx-lora-gateway/firmware/GA6.2/fw.txt