"Would there be much inefficiency in using an Antigravity battery (believe this is 18AH) to juice up the Goal Zero? Unsure of a way to connect the two directly. Thanks!"
Short answer: It would work, but the technique is very inefficient.
The GoalZero power supply is 15V @ 1.5A, which they say will charge the 5.2AH battery in a nominal 3 hours. I'm reasonably certain there's a voltage regulator in the Sherpa that limits the charging current to no more than 1.5A. So, while you could take the nominal 15V jump start output of the Antigravity pack and use it to charge the GoalZero, you'd be better just using the Antigravity pack to power your electronics.
I might add that while GoalZero makes a very good product, many of us bought them before the introduction of the Antigravity batteries. When I upgrade systems, I'm going with Antigravity.