Whenever I try to change the graphics settings I get a message that the game is not installed.

I installed the game using run as administrator. The game runs fine but I'd just like to step the resolution up a bit. Any ideas?

Mind linking that thread

Might be these ones -
http://www.gog.com/en/forum/syndicate/fullscreen_v_windowed_mode
http://www.gog.com/en/forum/syndicate/no_video_ingamecant_change_settings_in_graphic_mode_setup

But I think I found the issue, though it doesn't make sense to me -

If I run the file 'Graphic mode setup.exe' from the location 'C:\[your chosen location]\GOG\Syndicate\' folder I get the error Game not installed

Hoever if I run the file from the start menu shortcut which uses this target path -

"C:\[your chosen location]\GOG\Syndicate\Graphic mode setup.exe" GOGSYNDICATE

I don't get the error. So either try running the Graphic mode setup from your start menu shortcut or create a new shortcut with the above target path.

Let us know if it works or not.