I have a similiar question: I want to go the other way round, and make monsters NOT possessible by the GM. According to your comment I tried the following gameScript, but it does not work (I used an inventory item tagged with "POSSESSABLE" to mark characters I would like to be able to possess):

Code
INIT

EVENTS
EVENT CombatStarted
VARS
	CHARACTER:_thisChar
	ITEM:_possesMeItem
ON
	OnEnteredCombat(_thisChar,_)
ACTIONS
	IF "(!c1)"
	    ItemGetFromInventory(_possesMeItem, _thisChar, null, "POSSESSABLE")
	THEN
		//DisplayText(_thisChar, "NOT possessable start", FLOAT:2.5)
		SetFlag(_thisChar, "CHAR_MULTIPLAYER")
	    CharacterItemEvent(_thisChar, null, "NotPossessable")
	ENDIF


I also added a story script (via that CharacterItemEvent call) and added the not possessable monsters as players via

Code
IF
CharacterItemEvent(_thisCharacter, _, "NotPossessable")
THEN
DB_IsPlayer(_thisCharacter);


This way, monsters were indeed not possessable anymore, but the AI was also disabled and the monsters did not attack anyone.

Any help?