This short story is fairly representative of Gerald Durrell's entire body of work. Like his other stories, it is a non-fiction story written in the first person, largely autobiographical, written in a humorous tone of voice.
When critically analyzing anything, you should consider the purpose behind the text—what is the author trying to convey, and what is his underlying message? The humorous tone of this story indicates that a large part of Durrell's purpose is to entertain, but the themes of the story suggest a broader purpose too. A naturalist with a great and lifelong love of animals, Durrell seeks also to educate and to share his love of animals with others. At the beginning of the story, when Durrell agrees to take the gorilla, despite the fact that he really cannot afford to pay for it, he explains that the many "newly emergent governments" in Africa at that time had little interest in preserving gorillas, and Durrell feared they "might well become extinct within the next twenty years." While superficially, this story is about one particular gorilla, it also seeks to raise awareness of the plight of gorillas more widely.
This brings us, then, to the theme of the story, something key to a literary analysis. One of the key themes of this story is the relationship between humans and animals, and the responsibilities humans have to preserve and look after our animal friends. The language Durrell uses in this story while speaking about the gorilla convey the idea of a kinship between it, or "him," and Durrell. Some of the words Durrell uses to describe N'Pongo—"handsome," "giggling," with eyes "twinkling"—would be more usually used to describe a human. Durrell forms a close bond with his gorilla; it is not just an animal to him, but becomes a close companion. This is most evident during the point in the story when N'Pongo is sick, and Durrell fears he will not survive. Durrell and his wife go to great lengths to alleviate N'Pongo's "emaciated condition," and although his illness seems beyond hope, eventually the dedication of Durrell and his wife proves successful: their love for the gorilla saves its life. Given the wider themes of the story, we can interpret this as an indication that with proper human attention and dedication, the lives of the wider gorilla population could be saved as well.