Just wondering how do I play Settlers 2 multiplayer?

Has anyone done it with the GOG version?

there's a splitscreen option, which requieres a second mouse. the searches at the com-port, not sure how it works with the gog version. the screen will be splitted vertically with every players coursor stuck to his/her/its side. 1024x768 as resolution is highly recommended, otherwise you will have a hard time navigating through your settlement ;)
an alternative: try RttR (Return to the Roots - www.siedler25.org ), an rebuild of the game which adds network multiplayer. It's not finished and sometimes very unstable, but give it a try.