Can you customize resolution e.g. 1366x768?
There are system files you can edit with a text editor, but I can't find the entry for screen resolution.

Unofficial patch, including widescreen here. After installation you should find 1366x768 available in the game's options.

Note that the GOG version comes with a couple of annoyances: First, It makes the game run as administrator, which is unnecessary and will break vsync and the brightness setting. Second, it prevents you from accessing the desktop using alt-tab.

Read the "additional notes" section in the linked post for information on how to fix those GOG issues.