Technically, you actually can finish the game without it. There is only one planet that you need to go to in order to finish the game and its map file is not overridden.

However having said that, take a look at the U2 Upgrade. It adds the galactic maps, upgrades the graphics to EGA, introduces restore and exit hotkeys, and fixes many bugs with the original:

http://exodus.voyd.net/ultima2.html

The install process isn't streamlined for the GOG releases, so if you run into trouble use Pix's Ultima patcher:

http://www.pixsoriginadventures.co.uk/ultima-patcher-1-2/

Hope that helps.

Answering the four years old question in case somebody stumbles on it, I've completed the GOG version of Ultima II without using any other "upgrades" or patches. The game is perfectly winnable, and one can visit all of the planets.

there's a bug in the pc version???
i never encountered it back on my original U1-6 dos cd

that would have so pissed me off back then if i had encountered itXD

The original PC vrsion came on multiple floppy discs, and some of the map files on them had the same names. The first CD version was made by just dumping all the files into one directory and hacking the game exe to skip the drive swap prompts, resulting in the maps for the non-plot critical planets being overwritten by the maps of the Earth time periods. The critical planet map was still working however.

As a result there was a fan patch created to fix the issue, providing the correct maps with new names and patching the game to use the new files. The GoG version of the game appears similarly fixed. Judging by the file names in the GoG U2 directory I assume they used the fan patch to do it.