G'day. Best practice is to always use the firmware version which is running on your unit at the time to extract the boot.bin [ldr.bin]. If your current fw is v8.20 then the gupdate.gcd in .System folder will be exactly the same file as 'nuvi34xx_GCDFile__820.gcd' and so either can be used. Compare the hashes and/or file size in bytes [77,757,239 bytes].