You'll find several gardens and a greenhouse managed by robots in Graygarden. You won't encounter any enemies whatsoever, neither will you find any valuable items. You can, however, perform some activities.
You can trade with Supervisor Green. He does not possess anything of special value.
While talking to Supervisor Green, he'll ask you to follow him. He wants to give you a reward, but it turns out to be an old car wreck. You can get some bottlecaps from him during the second conversation. Depending on your charisma you'll receive from 100 to 200 bottlecaps.
Supervisor Brown can buy from you fruit that you harvest on the farm. Each one will give you 2 bottlecaps.
Supervisor White can give you the Troubled Waters quest. When you complete it, you'll get support of the robots that work here, you'll be able to turn the farm into your settlement and collect all valuable items here.