From Grand Theft Wiki
Guernsey City is a district located in New Guernsey in Grand Theft Auto 1, based on Jersey City, New Jersey. The district is bordered by Hackenslash to the north, Fort Law and Schlechberg to the east and water and New Guernsey area to the South.
- Jimmy Pegorino mentions to Niko Bellic that The Commission think's of Pegorino's crew as "just a bunch of Guernsey goombas" implying that Guernsey City has crossed over from Grand Theft Auto 1 to the Grand Theft Auto IV Era (much like Liberty City, Vice City, and San Andreas). It may be a city within the Alderney State. Since only one island in the state is present in Grand Theft Auto IV and it's Episodes.