The United States has covered some of their states in the WTO GPA, 37 states; however, there are a lot of exclusions that apply to those states. They are able to administer the Buy American program, which is a federally instituted policy that applies to transfer payments from the federal government to state or municipal governments. As a condition of receiving those federal funds, they must do Buy America in terms of buying American things. It depends on the types of procurements that are being carried out. If they are infrastructure projects, it has to be American steel, for example.
They are able to do this within their trade agreements because even the GPA includes a lot of exceptions, including for construction-grade steel. These are exceptions that would have been taken by the various states. Not every state has an exception for construction grade steel, but many do. Most do.