Bugs take many many generations to become resistant to chemical pesticides so that is just bro-science my friend. Preparations of bacterial pesticides and things like insecticidal soaps/oil sprays never breed resistant bugs.
Spinosad is for spraying on leaves is it not? Gnats don’t live in the canopy so it’s kind of useless for them. You might kill a few but they are as likely to be walking on the ceiling as sitting on a leaf. Adult gnats just fly around and mate then the males die and the female lays her eggs in moist soil. Yellow sticky traps just above the soil surface will trap the majority of adults soon after they emerge and things like the dunks will kill off most of the larvae. I have some stuff called Gnatrol I bought a while back but haven’t had need yet so not sure if it works but nice to have handy. If I can find it. lol
After some research I can find no mention of Spinosad being effective against fungus gnats or any other soil dwelling pest. All sorts of things that attack the leaves but not gnats. Think I’ll get some for those damn thrips tho.
It’s man-made chemical 'cides that bugs and weeds get resistant to eventually in much the same way we’ve ended up with all sorts of antibiotic resistant bacteria that now plague us. It still takes years and like I said, many many generations before resistance sets in.
I got rid of mine by using lots of sticky traps and a good lot of diatomaceous earth on top of the soil and kept that dry by watering from the bottom. I also blocked the source of the bugs which was the sump right beside the grow room and actually part of it. Bags of soil are very often the source as they will infect bags that are waiting to be sold and have even tiny holes in them. They can also infect soilless mixes too so keep all that stuff far away from the plants.
Even if you just keep the population low then they aren’t going to hurt the plants much but best to get rid of them.