If you want to change the size of the playable area in Game Guru you can modify the dimensions of the box inside the gameplayercontrol.lua file:
-- Prevent player leaving terrain area
if ( GetPlrObjectPositionX()<100 ) then SetGamePlayerControlPushangle(90.0) SetGamePlayerControlPushforce(1.0) end
if ( GetPlrObjectPositionX()>51100 ) then SetGamePlayerControlPushangle(270.0) SetGamePlayerControlPushforce(1.0) end
if ( GetPlrObjectPositionZ()<100 ) then SetGamePlayerControlPushangle(0.0) SetGamePlayerControlPushforce(1.0) end
if ( GetPlrObjectPositionZ()>51100 ) then SetGamePlayerControlPushangle(180.0) SetGamePlayerControlPushforce(1.0) end
By default the terrain size is 51100 x 51100
This however is all in theory as ive never modified these values before to see their result.