SantoRichter, nice to see someone on the same boat as me, I have not come across many GSSP nominees so far.
In my case, the Graduate School did not say anything about a possible deadline; they only explained that this process is more of a formality but time-consuming nonetheless. They also said that the portal had gone through some major changes this year (for instance, it did not require the reference letters) and that they would contact the DAAD for instructions.
I uploaded my documents to the DAAD portal on 23 April, and as of today there hasn't been any change to my status. It may be a good idea to ask the Grad School about an approximate time scale, which I will do on Monday. The end of April-beginning of May deadline has largely passed, have you contacted the committee?
My PhD programme is at the FU Berlin.
Best of luck.