Surely that depends on what was in the moussaka and how much you ate.
If it is home made rather than from a packet (which would have the carb content), it may have potatoes it or it may not, it may have flour in the topping or it may not, it will probably have tomatoes but that varies, I wouldn't call it a moussaka unless it had aubergines but how many is up to the chef.
Like all food that does not come with a label it is guess work based on experience.
But, whatever the recipe, moussaka is high in fat so there may be a delay in the absorption of the carbs.