you have flower; it's flowering. why it took 13 weeks to get this far is another question with several potential answers and no certainty, lol.
go soilless.. .skip the learning curve. check out guides and articles on cocoforcannabis.com. best resource for non-broscience information. mega crop, crop salt, jacks 321, southern ag, masterblend all have an excellent 3-part fertilizer setup. some 'base', calcium nitrate and epsom salt (a.k.a. magnesium sulfate). Two of those you can buy cheap generics as long as the brand doesn't put the trace elements into them. The greedy brands do that.
gnats -- Look into Bti, never have a gnat problem again... 1 or 2 random gants? maybe, but never gets out of control. Gnats come with the soil more times than not. During transport or stored outside it often picks up gnats at the least. Bti is a bacteria that is perfectly safe to use. top-dress 'bits' or reservoir 'dunks' (floater) options. should kill just about anything that forms larva in the medium. Can use it pre-emptively.. .if no sign of gnats several weeks into grow can stop using it.