G-Infinity

G-0, officially known as the Marine Grand Line 0th Branch (海軍G・L第0支部, Kaigun Gurando Rain Dai-Zero Shibu), is one of the various G-bases under Marine control — yet it's noted to be also under the supervision of the World Government, having incredible importance to them because of the research materials and other top-secret projects held here; it's known as the "Phantom Marine Base" (幻像海軍支部, Genzou Kaigun Shibu), it's location is disclosed to a select few and not truly counted as part of the G-base lineup. In addition, it appears more of a metropolis and a safe haven for many Marine scientists, thus also gains the moniker of "Marine Science Metropolis" (海軍理系首都, Kaigun Rikei Shuto).

Like, it's located on an island referred to as both an "afternoon island" (昼島, hirujima) and an "never-night island" (不夜島, fuyajima). Due to her contributions to the Marines and the World Government via MARITECH, it's given to and controlled by Ganymede Pyrrha — continuing her research in peace while giving her a variety of materials and help as she sees fit.

Trivia

 * This is based on the First Magitek Production Facility from Final Fantasy XV, Academy City from the To Aru series, and the Metropolis level from Sonic Heroes and Sonic Forces.