Recommended to pour an inch or so out of the milk container first, because it will expand and maybe burst.
We've never had one burst, but they bulge a lot. And pouring a glass out might prevent this. After thawing out, shake vigorously for about 10 seconds and make sure to hold the lid on with one hand. Our last gallon is thawing out now - so we have to make a run to the grocery soon - not just for milk, but also for some fresh food items.