For sure, there is no size fit all when it comes to withdrawal methods, or at least, I doubt there is. As usual, one has to ponder about your goals in selecting what works for you. The G-K method, when stripped to its fundamentals (essentially stay centered on a fixed % of your
current portfolio, with those guardrails to make you smoothly adjust your withdrawals - as nicely explained by archbish99), will work very well for some folks, or not for others. And it's really quite trivial to program in an Excel sheet or equivalent.
If you wish to spend all your money, or if you can't deal with some variability in your spend, or if you have all the money in the world, then forget it, there are better ways. But if you aim at keeping your portfolio value somewhat stable (on average) for an indefinite time, can deal with some spending variations but not too much, and would like to take advantage of the ups of the markets while dealing with the downs of it, then this is a very sensible method. And to be clear, it DOES adjust to grimmer future returns than what occurred in the past - or better ones, actually. Again, this is centered on a fixed % of your
current portfolio, not your initial portfolio, so if things go bad, well, withdrawals will shrink accordingly.
To make your own assessment, there is at least one retirement calculator that implemented a basic form of the Guyton-Klinger "guardrails":
http://www.cfiresim.com/input.php. One can play on the initial spending to indirectly affect the % of the portfolio that the guardrails are centered on. And one can play on the fees (try 1% or 2%) to model much grimmer returns than actually occurred in the past.
If you play with it long enough, you'll see that the smoothing mechanism is really quite good, not only for the immediate effect of avoiding jittery effects on your spend, but also avoiding to overreact to market gyrations and be there for the long run. Which I found crucial to do, actually. This is where the flat 4% of the initial portfolio method does convey a bit of wisdom, it can take 20 years for things to return to 'normal', and going through crazy jitters in the meantime will do nothing good for your blood pressure, and little to help your portfolio. I don't agree that this doesn't properly address big crisis (or big bull markets), by the way. Just run simulations with sensible inputs, and you'll see.