GTA Plans (ai_gta_plan_set)
DO_NOTHING
DEFEND_FEATURE
ATTACK_ALL
ATTACK_SETTLEMENT
GTA Objectives (ai_gta_add_objective)
ATTACK_ENEMY_BATTLEGROUP
DEFEND_LINE
ASSAULT_CROSSING
DEFEND_TERRAIN_AREA
MOVE_TO_POINT
Things I wish worked but was never able to get working (invalid plan apparently even if it is listed in the exe)
HOLD_AND_SHOOT
DEFEND_SETTLEMENT/DEFEND_CROSSING (Likely merged into DEFEND_FEATURE though I just leave off trying to script any sort of AI defense in settlements as you can have some very adverse results)
As for what GTA stands for generic troop assignments (?). Never figured it out either, anyway @UndyingNephilim I don't check this area very often so if you stopped by here the next time you had any script/AI related questions myself or others (for example Granto/CavalryCmdr) would be happy to attempt answering them.
Also I highly reccomend (despite the self promotion) reading through this tutorial it has information I completely skipped over in regards to how to use the above commands.