Only proceed if you have a copy of the original firmware to roll back to. Look for .bin or .img files specifically tagged for the GM219-S.
However, for a device designated "GM219-S" (often associated with the suffix 'S' for 'Super' or specific PON speeds), it is most commonly deployed in mode using the Realtek chipset's emulation capabilities. firmware gm219-s xpon
Most GM219-S units run a customized version of the , usually running a Linux kernel (often version 2.6.x or 3.x depending on the age). Only proceed if you have a copy of