Getting to Gorgona takes effort, but the remoteness is part of the appeal.
By air and boat via Guapi. Fly to Guapi on Satena (routes from Bogota and Medellin), then rent a motorboat for the roughly five-hour crossing. Boat rental is priced per vessel (capacity 10-15 people), not per person, so small groups pay more.
By cargo boat from Buenaventura. Buenaventura is easily reached by bus. From the Piñal dock (Muelle del Piñal), cargo and passenger boats (barcos de cabotaje) run daily to Olaya Herrera and will detour to drop you at Gorgona. The trip takes 10-12 hours. Book ahead or negotiate on arrival.
Diving trips from the mainland. Dive centers in Cali, Medellin, and Bogota regularly organize group trips to Gorgona. A good option if you prefer someone else handling the logistics.
In all cases, wrap your bag in plastic. It will travel on deck and arrive drenched. Book accommodation with Parques Nacionales Naturales (parquesnacionales.gov.co) well in advance -- the island has limited capacity.