You may gain just fine on that program, you may not. In any case, 20-rep squats are probably the most overrated training protocol ever. The 20 rep squat madness started in the 1930s with a man named JC Hise. He gained 29 lbs in a month doing 20 rep squats and drinking milk. His only other exercise was the press behind neck.
You can read the initial writeup about him (from the 1930s) here:
It starts talking about him near the end of the first page. (None of the photographs in the article are him, in case you don’t read the captions.)
Now here’s what you’ll notice if you read carefully… It talks about how his measurements changed… chest increased, arms… but WAIT. Waist, from 36 to 41 in a MONTH. He gained 29 lbs of almost all fat and they acted like it was a great accomplishment and people should emulate what he did. And from that beginning 20 rep squats became popular.
The article also talks about how he did “reducing exercises” (ie, abdominal work, under the belief that these exercises would keep fat off his belly), but stopped because he believed they were decreasing his height.
The simple fact is that when the 20-rep squat + milk protocol was created, it was still fairly early days in the iron game, and people were enthusiastic but ignorant. If I were you, I would look elsewhere for a good routine.