The tool is written in "C" and must be compiled adding a file (gpib-32.obj) for the used GPIB adapter.
If you have trouble doing it I can attach a version compiled for NI GPIB adapters, it definitely works for NI GPIB-USB-HS and the old NI GPIB PCII/IIA,
I don't know if it works with other non NI adapters.
The command to be given to back up the NVRAM is:
tektool -r nvram.bin -b 0x04000000 -l 0xa0000
Where:
-r : Read command
nvram.bin: Backup file name
-b 0x04000000: NVRAM start address
-l 0xa0000: NVRAM file lenght
The starting address is right for TDS784D/TDS794D and my TDS784C.
Check on TDS620A service manual that the start address is the same otherwise you save the wrong data.
Good luck.