This is just an observation. 

I have on the same table some persimmon seedings in small pots and the fig cuttings in SIPs. Same soil. The gnats are really attracted by something near the figs but clearly do not care much about the persimmons. Do they smell the tasty roots?

Am I imagining or this is kind of true? 

I have some gnats in my orchids (growing in fir bark chunks) and some amaryllis I have growing on shelf above the figs.

Sprouting fig cuttings sure do stink. Maybe that has something to do with it?

Cinnamon again?

they are nasty creatures and live anywhere. I clean a super neat and clean house, there are no house plants there, no fruits on counters, and they are there, living inside their food disposal, shower drain, you name it... they, like cockroaches are adapdable, thus will survive us, in many years from now, the planet will be inhabited by roaches and gnats...