Except that, in English, it's actually pronounced very similar to gorilla (the vowels may be slightly different, but the r and l sounds are normal English). I speak Spanish, and I'm aware that's how you say it in Spanish, but that's not how you say it in English. If you want to say it that way, that's not a problem, but that's not the standard English way to say it.

Incidentally, the same applies to the meaning as well: In English, we use guerrilla as an adjective, and it can apply to either the war or the soldiers ("guerrillero", if you prefer). Adjectives can also be used substantively, so "guerrilla" on its own is usually used to mean a guerrila soldier. I recognize this is different from its usage in Spanish.

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...

You'd be surprised:

Guerrilla, noun a member of a band of irregular soldiers that uses guerrilla warfare, harassing the enemy by surprise raids, sabotaging communication and supply lines, etc.

Just because a word is written like in your language and/or is derrived from it, doesn't mean it's meant to be read as belonging IN your language.

It's not even metonymy proper.

It's the adoption of a spanish word in another language, in which it IS used as a noun in the first place.