That's weird, because searching "guerrillas" (an unambiguously nominal form of the word) in the Corpus of Contemporary English (COCA) gives me over two thousand hits!

http://corpus.byu.edu/coca/

And the Oxford dictionary defines "guerrilla" as

a member of a small independent group taking part in irregular fighting, typically against larger regular forces

and gives the IPA pronunciation as /gəˈrIlə/ AKA "gorilla".

It's almost like language evolves, and borrowed words don't always retain their pronunciations... no, it couldn't be...