Sgretola comes from the verb sgretolare, meaning "to crumble" or "to disintegrate". It's a vivid and somewhat dramatic word that isn't used in everyday conversation, making it stand out.
In the song, Capo Plaza sings, "Tutto si sgretola in fretta" (Everything crumbles quickly). He uses this word to powerfully convey the rapid and destructive breakdown of a relationship or situation, emphasizing the fragility and impermanence of things.
Capo Plaza built this song out of the last two minutes of an argument, the part where one person is already at the door and the other is still talking. The title line, "Non fare così" (Don't act like that), is what you say when you have already lost and have not accepted it yet. He keeps asking, "Baby, non andare, rimani" (Baby, don't go, stay), and then lays out the only two options he can still see: "Ne possiamo parlare o puntarci le armi" (We can talk or point guns at each other). As a threat that line is ugly. As an admission that talking and fighting are the only two settings left, it is closer to what the song is actually about.
The money is what makes it sting. "Prima rubavamo abiti" (We used to steal clothes) comes one line before "Ora ho camicie di Burberry" (Now I've got Burberry shirts), and he is proud of the distance between those two lines, but the success arrives too late to buy anything back. He knows the scale was wrong all along: "E tu mi chiedevi la luna" (And you asked me for the moon), "Ti meritavi, sì, tutti i pianeti" (You deserved, yeah, all the planets). By the last verse he has stopped talking to her and started talking about her, "E non so manco che fai, con chi sei, dove stai" (And I don't even know what you do, who you're with, where you are), and the only thing he can still put in a calendar is "a domani" (till tomorrow), an appointment nobody agreed to.