Hi,I found only two Joseph P McGrath's on the NARA and Ancestry sites that enlisted in Kings, NY.One was born in 1917 and the other as mentioned 1926.I've looked for family and family trees on Ancestry and couldn't find anything.
Is there a chance that the IDPF is coming?If so, that may help.They are available but it takes about 4-6 months to get.
I'm sure you know he is buried in Florence, Toscana, Italy in the American Cemetery.
He is not remembered on the WWII Memorial site by anyone either.
Best I can do you for now.Maybe someone else will have better luck.If you find out more information without actually finding family, please post it and I'll try again.