Yes you can freeze the milk in the plastic gallon jug but if you do freeze the milk in the plastic jug the jug might decide to break so be prepared for that just in case it happens.
Also the milk is still okay to drink after it's been frozen but do note that if you do freeze the milk it won't taste as good or fresh as it did before freezing it.
I personally don't like frozen milk but some people are okay with it.
Just don't freeze milk in the glass jars or glass containers because the cold frozen temperatures will break the glass because of the ice expansion too.