Hünkar Çayırı (Sultan's Meadow) is a picnic area run by Gebze district's municipality. 

According to some sources this is where Mehmed 'The Conqueror" (Mehmet II) set up a camp with his army on his last campaign before his death.

The area is very basic with a historic fountain and some benches. Nearby are also the remnants of a former automotive plant which is open to the public and essentially acts as an extension of the picnic area. 

I wrote about the plant in more detail in the other notes if you are interested.