The programme is no secret. There are many variations but the basic one is,
Monday - squat 5,5,5+; press 5,5,5+
Wednesday - bench 5,5,5+, deadlift 5+
Friday - squat 5,5,5+;press 5,5,5+
and in the following week it's the same except you swap out press for bench. "5+" means at least 5 reps, keep going until you feel the next one would be grindy and/or messy. Normal weights progression is 1kg a time on press or bench, and 2kg a time on squat or deadlift. Optionally, if you get 12+ you might double the progression next time.
It arose as a variant of Starting Strength because the authour was depressed by endless stalls and deloads; better slow and sustained progress than fast progress with setbacks, goes the reasoning. Whether it's physically better is up to argument, but I think it's mentally better. Thus, GSLP in its most basic form is basically SS with one less squat session a week, and the "as many" sets at the end.
From there he adds in "plugins" if you want more gunz or abz or are doing it for weightlifting or whatever.
My thought are these.
1. doing just two exercises a session lets you focus your efforts on them, rather than leaving something in the tank for a third, fourth etc as in other programmes. This is also useful for time-poor people.
2. Since you are squatting twice a week and deadlifting once, squats will get ahead of deadlifts unless you're doing quite high reps (12+) in deadlift, and
3. high reps in deadlifts risks injury, at least in newbies.
4. fatigue degrading form is an issue with every lift. A newbie needs to practice doing it right, not practice doing it wrong (as an aside, this is one flaw in vanilla SS - doing 3 sets of chinups to failure). Yes, it does say to stop when it gets messy, but to a newbie every rep feels messy, it's all new to them. So then really it's just up to the newbie's personality if they err on the side of wuss or on the side of meathead; one leads to less progress and the other to injury. But this really is an issue for anyone using any programme on their own, and is not a particular flaw for GSLP.
5. "as many" sets build volume. A newbie is lifting relatively small loads and thus can handle a higher total number of reps, and it probably does them good to do them. As the load increases the benefit will decrease, but the person will drop reps anyway (from from 12-15 to 6-8 or even 5).
6. "as many" in combination with small jumps builds confidence. The newbie doesn't know what they're capable of, if they did 60kg for 5,5,5 they may be in doubt about hitting 62.5kg for 5,5,5; but if they did 60kg 5,5,11 they're not worried about hitting 61kg 5,5,5 at least.
So... fatigue degrading form vs volume & confidence. Which side the balance comes down on is open to argument. I would say that if it's someone's first time with barbells and they are not co-ordinated etc, I'd err on the side of less reps and something closer to vanilla SS. If they've had some barbell experience and/or are co-ordinated, the volume and confidence win out.
I have done a couple of runs of a variant of this. For reference, I am 45yo with a few old injuries, and two small children. I train people out of my garage gym. I just alternated bench/squat with press/deadlift, and stuck to the +1/+2kg jumps regardless of reps done. At 100kg for squat and deadlift I turned the 5+ into an ordinary 5, as the squats were taking too much out of me and deadlifts, well I have back problems already. After a while I dropped it to 2pw and did something else for my third. I have done this for 12 months out of the last 18, with some other stuff spread in between, best lifts squat 165, bench 110 and deadlift 215 for singles.