lots of misinformation here about the current state of play with respect to GTO. If you're interested in understanding the principles of GTO and how amateurs currently leverage it check out the Finding Equilibrium youtube channel. Specifically, there's a series where this guy teaches his girlfriend to play poker by building heuristics that generally mimic solver advice. It's a cool way to see how solvers are changing the live poker game; from this relatively basic series you can extrapolate how professionals refine their strategies and heuristics to get very close to solver approval in the majority of situations.