As Rotorhead says, if you already hold a "stand alone" FAA certificate, you need at least 3 hours in gliders, 10 solo flights and 3 flights with a CFI-G within 2 months of your checkride.
If you have a 61.75 certificate based on your German license and you have added a Sail Plane License to your German license you can have the glider rating added to your FAA certificate. If you do it this way and the Germans still require a medical to fly a glider, you'll have to have a medical to use your FAA certificate. You can use your German medical.