As far as I know, it should be on the market at the end of april, at wog it was scheduled for 6.3, now it is 30.4.

According to Sennheiser itself, the headset will be available from mid-May.....

Funnily enough, they are already available on Neptun (student shop). However, they cost more there...

Direct support can certainly tell you more.
But if you don't want to wait until the GSP600s come out, I can only recommend the Sennheiser Game One / Game Zero. Sennheiser GAME ZERO (Over-Ear, Black)