Guerrilla is not a type of person, it's a type of war. A "little war" to be exact. The person fighting in a guerrilla is a guerrilla soldier or a guerrilla fighter. Also, it's pronounced geriya (with a hard g and a rolling r), not gorilla.
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.
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...
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 the adoption of a spanish word in another language, in which it IS used as a noun in the first place.