Hi Susan,
Hnat is the name in Slovak but was spelled Hnath in Hungarian as that was the country that controlled that area in those days in Europe. Same thing happened with my grandfather. His name on his marriage certificate in America was Hnat but became Knott because that is how it sounded in America. I found some information for you about your grandfather Joseph.
According to the 1930 Federal Census, Joseph Hnath, age 52, lived with his wife Veronica, age 46 with their 9 children in New Jersey. If the information on the census is correct, Joseph was born about 1878 and Veronica born about 1884. He immigrated to America in 1891 and in 1930 was working at an iron foundry. He was age 24 when he married Veronica and she was age 18. They both gave their place of birth as Czecho-Slovak and the place of birth of their mothers and fathers as Czecho-Slovak. This would be the Slovakia of today where my grandfather also came from.
Hope this is helpful to you.
Dave