The Song of the Humpback Whale by Roger Payne. The scientist who discovered that humpback whales sing — and that their songs spread across ocean basins — describes the research that changed how we understand marine mammals. Payne's fieldwork was conducted partly in Pacific waters, and reading it before standing on a Gorgona beach and hearing whale song through the water gives the experience a different dimension.
Islas del Pacifico Colombiano by various authors (Banco de la República, Colombia). A detailed natural history of Colombia's Pacific islands, including Gorgona, with sections on geology, endemic species, and the transition from prison to protected park. In Spanish, but accessible to anyone with basic reading ability in the language. The photography alone is worth hunting down a copy.
El año del laberinto by Tomás González. A Colombian novel set partly on the Pacific coast, in the jungle and river towns that connect the interior to Gorgona's world. González writes about isolation, water, and the particular quality of Pacific-coast Colombia with more precision than any travel account. Not set on the island, but it puts you in the right frame.
The Human Shore by John R. Gillis. A broad history of how people have related to coastlines and islands throughout history, including the roles islands have played as places of confinement, exile, and escape. Puts Gorgona's prison history in a longer frame — the island as a tool of power — and is readable on a boat or a beach.