Set gravity to -200 instead of 200? Haven't tested, seems to me it would work :)
Yes! you are correct, just setting the object's world gravity to a negative number will work. To make this streamlined, the action within the event causing the gravity to change just needs to be this "On Gravity Change > Action > Object/Sprite > Set World Gravity to "worldgravityVariable * -1". This will toggle it's gravity every time this event is triggered from a positive number to a negative and so on.