What happens if the gfr drops, what will the kidneys do via autoregulation to bring gfr back to homeostatic range?

Whole body stuff: Don't focus on today's subscientific estimates of GFR that you get for free with routine bloodwork. If GFR drops and you keep drinking water, blood volume rises and GFR goes back up. The macula densa may sense more concentrated glomerular filtrate and generate renin; you know what that does. There are a bunch of other minor mechanisms. It works pretty well for the minor stresses of life and sports