It is down. I confirmed this with myself, a friend of mine, and now with you indirectly.
The reason for this is unknown. It could possibly be the “Forgot Your Password” tool that did not work earlier, but in reality I don’t know and I don’t think many people know.
It should be up soon. I’m sure USACO understands the importance of the Gateway. For now, I suggest you create test cases yourself and test those out with your code. It’s an important skill that’s incredibly useful for when you participate in the competitive programming contests.
It was down before for server maintenance, but came up I believe on the next day. I assure you it’ll be up soon.