I have a GB40 as well and have used it to start several different cars, Nissan Quest, Toyota Camry V6, BMW 535i. With the little wires and the wimpy guts (based on AvE’s teardown) it seems like it wouldn’t work but it does.
On the Camry I had previously tried a Costco jump pack (generic name) and it make the lights come on but wouldn’t turn over quick enough to start.