As slang certainly, and rather rude slang at that. In my experience, it does not mean to ramble, which is too laid back an activity. To spill one’s guts (to me) is to tell everything, and usually in some sort of confession, forced or voluntary.
Here is Wiktionary’s effort:
[i]spill one’s guts
- (slang) (idiomatic) To confess, or to divulge secrets, typically speaking freely and at length after a change of motive or an incentive.
At first, the suspect would not tell us anything about the other participants in the crime. But after being offered a deal, he spilled his guts.
And here is a BrE definition:
Vrb phrs. 1. To confess or reveal the truth.
2. To vomit. Probably a misuse of version 1 due to its similarity with ‘spew one’s guts up’.
And an AmE / OzE one:
spill one’s guts v. Confess all.