> we are waiting for a GCN1 driver.

Some X5000 or Sam460 owners are, yes.

> GCN2 and GCN3 were skipped by OS4 in favor of Polaris (GCN4). While
> GCN5, Vega, is already available and what may be the last generation,
> GCN6, Navi will be out this year. Is this a correct summary?

I think so, yes.

> Still on the basis of availability, and the pricing of lower end
> GCN4 cards, maybe the jump makes sense.

I think the main incentive for the MorphOS team to jump to GCN1 or even GCN4 should be the prospect of gaining a foothold with the owners of boards/systems MorphOS was ported to latest.