Good question, Mozzer.
All autobiographies and "tell-alls" are self-serving. Mobsters have more motivation than any others to embellish. They're breaking omerta and therefore have to "justify" their violation of their "oath" by whipping up a rationale for being a rat.

Probably a lot of the circumstantial stuff Gravano revealed was accurate because, in order to get his plea-bargain, he had to solve murders that the Feds who cut him his break needed to have solved. So, I believe him when he said he did or was complicit in 19 murders--those murders were on record. I also believe a lot of the construction rackets he talked about--standard Mafia stuff, and he was a very entrepreneurial gangster. I also believe a lot of the stuff he said about Gotti because Gotti was a publicity hound, and it was in the public record.
But there's a lot of BS in there, too. For example, he derides Castellano as a mere "racketeer," while he and Gotti were (gosh!) gangster! Yet he repeats every kudo the hated Castellano ever threw his way. And he brags about his "loyalty" to "Mafia," meanwhile jumping to the Feds' side as soon as it looked like Gotti was going to exercise his Don's prerogative by making Sammy fall on his sword. Some "Mafia" guy.