There is some damage on the first set of leaves, I...

I'm an inexperienced grower, but I know from "the books" that the problem with fungus gnats is not the little fuckers themself, but their offspring: larvae living in the top 2-3 inch of the medium having a party with the roots of your plant. Let the toplayer of the medium dry out for the first 2 inches before you water/feed again a few times and they're gone. If its not the gnats, than it must be a deficiency of a mobile nutrient. Calcium is immobile, hence its unlikely only the lowest leaves will manifest the deficiency, in fact, it will most likely show on the newest and most light exposed parts first. A mobile nutrient gets sucked out of the lower leaves; basicly the plant starts eating itself. When its the lower parts of the plant that are affected first, if it is a nutrient deficiency, it most likely is a mobile nutrient. However: your feeding schedule looks good, it should be enough, therefore a lockout is more likely (again, if its not the gnats larvae). Maybe the pH was too high and locking out some nutrients? Last: are you mixing nutes correctly? Shake every bottle before use and there is a certain sequence you should follow. If you only use GH trio, start with adding Micro to the water, then stir before adding the next component, stir again and then the last component. I believe calmag must be added before the trio (thus also before micro), but google around to find out more specific info. Assuming you're mixing correctly, I'd try to keep that pH around 6, keep on measuring pH of the runoff and adjust pH of feedingwater when nessercary (f.e. if runoff = 6.5, while 6 goes in, than next time, pH feeding water down to 5,5). I'd also let the top 1,5-2 inch of the medium dry out between every watering for at least 4 cycles to make sure all the larvae are dead. Hope this gives you some inspiration. Goodluck!