A short boost with 12V isn't likely to hurt gauges, etc. but there is some danger involved. As soon as the jumper cables are connected, the 6V battery will receive a quick burst of current which will make it generate a lot of Hydrogen gas. A spark near the top of the battery can ignite that gas, causing an explosion. To avoid problems, connect the first cable to the "hot" side of both batteries, but make the final connections somewhere else, such as engine block to engine block, etc. Any spark that gets generated will be away from the batteries. That's also a good rule of thumb to follow when both vehicles are 12V. Also pay attention to ground polarity- - - -most GMC's are Positive ground so it's easy to get the polarity reversed. That really creates some fireworks!
