First of all, what you are referring to are firmware updates, not drivers. Firmware updates actually modify the code on a chip in the drive. This code determines how a drive behaves in many ways. Drivers are bits of software which tell the operating system exactly how to talk to the drive to get it to do specific things.
Unlike drivers, you have to use exactly the same model of firmware updates as the drive model you have. You cannot use something close to, but not exactly the same model as your drive's model. The difference between two model numbers can be that the hardware inside is entirely different. Flashing with the wrong firmware can kill your drive, and invalidate warranty cover as well. So no, don't use the H20N's firmware.
From what I know, the GSA-H21N is an OEM only model, meaning it is a custom model for OEM computer manufacturers (e.g. HP, Dell) who bundle the drives with their computers. Due to this fact, LG does not provide firmware updates on their webpages for OEM models - it is instead left to the OEM to provide the firmware updates. You'll have to look for whichever computer manufacturer made the computer which your drive came preloaded in to find firmware updates.
Why is a GSA model an OEM model in this case? I don't know. LG is doing funny things again with their current model number system.