I'm using GG for one year and a half now and my overall feeling is that it is a great and easy scene maker, able to compete with professionnal studios if skilfully used (check Wolf's WIP thread to see what kind of wonderfull scenes a talented artist can achieve with GG) but GG is clearly unappropriate to make a game beyond basic FPS. What I mean is that you can easily make game with very limited environment interactions and story line such as Half-Life, Wolfenstein, Medal of Honor or Project IGI if you need references but not much.
It's my opinion that, for an indie game (GG is an indie game engine for indie game makers), story line and environment interaction do actually the difference between a total failure exposed to harsh critics by reviewers (steam, You Tube...) and a game worthing a try.
So what about a Mission Editor Mode ?
I often quote Bohemia Interactive's Operation Flashpoint in my posts ( sorry about that) but this game (wich dates back to 2001, neolithic age in video games' world) had a powerfull and extremly user-friendly Mission Editor to be inspired from in order to boosten GG potential.

It features :

Waypoints : Ability to toggle AI's speed (walk, run ...) and behaviour (unaware, cautious, combat..), set message text (radio call, conversation...) and play sound/music

Triggers : Ability to set condition of activation (presence/non presence, spotted by Friend, Foe or neutral, variable value), ability to set variable value and variable condition (simple IF... THEN coding), set message text (radio call, conversation...) and play sound/music

Synchronisation : Ability to synchronize Waypoint/Waypoint, Waypoint/Trigger by simply dragging a line between synchronised markers ( an example of use : a bunch of villagers wander around their house and field ( path of 2 waypoints set to normal walk mode and unaware behaviour for each villager. Their third waypoint is far away from the village and set to run mode. You close the village, activating a trigger setting alarm value to 1. A second trigger activated by alarm value == 1 is synchronized to villagers' second waypoint set to "Change". As a result, the villagers' path will ignore waypoint 1, waypoint 3 is activated and all villagers flee away...)

Cut scene : Ability to launch a cut scene from waypoint or trigger with camera settings options ( different heights, static, moving around waypoint/trigger, zoom in, zoom out...)

I strongly believe that such features would make GG as easy as it's intended to be and would open it to a wide range a valuable games being designed

Here is an example of a trigger edit panel in OFP. Should it be usefull to Lee, i can forward the Editor Manuel if any inspiration can be taken from it for future updates.