I've never been a fan of CTI-maps, so maybe I misunderstand something here but... In my world GUI means Graphical User Interface, i.e the first screen in OFP where you can chose between Single Mission, Campaign, Multiplayer, Editor, etc. I fail to see how that could be MP specific.
Do you want to create different choices at the MP selection screen (where you chose wich loon to play during the game)? Examples:
- Time of Day
- Skip Intro
- Dificulty
These are quite easy to make and they are set up with parameters, defined in the description.ext. I can guide you through the process of adding a "Skip Intro" to your MP-mission, but I'll have to do it from my PC at home (i.e this evening).