to make sure I've got this right

Not quite.

starting with the same database I can do either

a) gbak with gc

Won't advance the oldest active

b) gbak without gc and gfix -sweep

Will move the oldest active and will get a few more records than a)

and I will have the same final result

Best is gback with garbage collect plus sweep:  moves the oldest actives, gets the most old versions, shouldn't take much longer since the sweep doesn't do much.

Extending this further, how can identify which tables contain (or did contain) the garbage. 

gstat shows the number of versions present before you start.

good luck,