Just hook the 12v donor battery to the 6v dead battery only long enough to turn the starter.
Do not leave cables hooked up any longer than necessary to turn the starter motor. The battery will get hot and start to leak acid. I've been jumping 6v systems with 12v batteries for 55 years and have never burned out a gas gauge or anything else.

~~ Jethro
1954 3100
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1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020