they're somewhat similar in design theory. Other than that, they're different. The block is re-engineered, and larger. Cylinder spacing is different between the two because you go from a 76mm bore in the GA16 to an 80mm bore in the QG18. Stroke stays the same at 88mm. Head ports are most likely larger on the QG, but the bolt up for the exhaust manifold is the same. This is not to say you can use any GA header, because you can't due to the differences in the chassis and the likelihood the GA header won't flow as well as the QG needs.
The intake manifolds are different as well, i don't know if the QG intake mani will bolt on the GA or not.