That will fit the last 50 years of Gitzo heads like that, they also make a 3/8-16 one.
Those ballheads are fine, just no drag control so hold your camera when you loosen it. Old ones need muscle - or clean and relube and it will work better without over-tightening.
yeah, my new one needs muscle, things suck. Anyone want to sell me a kirk/arca/RRS ball head?
I have that head and like it for use with small cameras.