When you became a US Citizen you automatically lost your German citizenship. They will not automatically know, since the US Government does not inform them, but technically you are no longer a German citizen, so your passport is basically invalid. You will not be able to renew your German passport, at that time they would find out because the german consulate would ask to see your GC to make sure you haven't become a US Citizen hence lost your German citizenship.
There is no reason to do anything though, no need to tell them or surrender the passport. Just travel with your US passport from now on.
I also want to add that while in general Germany does not allow Dual Citizenship, an exemption can be granted if you can prove certain disadvantage and continous strong ties to Germany. However, such an exemption needs to be granted before you take to oath which doesn't seem to be the case here.