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Gel batteries (and AGM) CAN outgas, particularly during overcharge. They don't in normal operation, though.
As for the connect-to-ground advice, I've tried it. On most cars I've tried, it doesn't work; battery to battery is the only way to do it. Positive on good to positive on dead, negative on dead to negative on good, in that order.
Connecting to the starter housing or other well-grounded thing should work too, but getting to those parts is more of a hazard than a possible spark.
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