I've come across a fairly frequent claim that gn was pronounced as ɲn in vulgar Latin. However, I haven't been able to find any kind of academic sources to back that up. I've found a few sources (and questions answered here) specific to Classical Latin, but none for Vulgar Latin.
I hate to put forth something as a known fact without having any kind of evidence to back me up, so I'd like to know more. Are there any good sources for this claim? How do we know that gn was pronounced as ɲn in vulgar latin?