I see that there are yellow leaves with brown spot...

Looks like a calmag issue, and also possible pH issue. In soil I add mycorrhiza to help keep the pH down where it matters, the myco also helps deal with the calmag as well. For starters, balance your pH, and add some calmag after. As for the gnats, if their population isn't large, then consider what is acceptable levels of pests instead of erradication. Soap sprays should be avoided in all stages of growth save for a last resort. Rootshield, advanced nutrients 1+1 tarantula and piranah will help protect the roots. As for harvest, it all in he trichomes... And your preference. When the tops of the trichomes are cloudy, THC has peaked, at Amber you end up with a heavier couchlock buzz, with more clear than Amber you have a headier buzz. All the while aiming for Max cloudiness. My experience with Jack herer strains is that they finish in weeks 8-9 of flower, but it also depends on the pheno. And you're growing an auto so best advice is get a loupe or mini microscope with a 30-60x zoom and watch the trichomes.. remember to check the lower canopy development as well... Happy gardening!