There are a few grammar mistakes in the passage. For the sentence you've quoted, there are a few different ways you could correct it. To stay close to the original sentence, you could make one of these two changes:
"True to gorillas’ unaggressive nature ..." (Here, I have put the apostrophe after the plural "gorillas," instead of between "gorilla" and "s." This means that I'm talking about the non-aggressive nature of "gorillas" in general, rather than talking about the nature of a single gorilla. This phrase is grammatically correct, but it sounds a little clumsy.)
"True to the gorilla's unaggressive nature ..." (Here, I've added the word "the," because the word "gorilla" is singular and needs an article. This sentence could be interpreted as talking about gorillas in general, if you take "the gorilla" as referring to a category of animals, rather than to one gorilla. It could also be interpreted as referring to the nature of this one particular gorilla, though, so it's a little unclear.)
If you want to make it clear that you're talking about gorillas, in general, being non-aggressive, then a longer phrase would work better. "True to the non-aggressive nature of gorillas, the huge animal meant me no real harm."
The first part of the sentence is saying that this gorilla is being "true to" its nature, meaning that it is doing what is natural for gorillas (being non-aggressive).
(I've replaced "unaggressive" with "non-aggressive," because "unaggressive" isn't really a word, although English speakers can understand what it means.)