There is no such thing as recall
Greyhounds are sight hounds, easily distracted and with a genetic predisposition to chasing. They can also hit a top speed of 75 kmh in four strides and within 45 seconds be almost one kilometre away from their frantic owners.
Once they decide to run there is no such thing as recall with a greyhound and therefore, like all sight hounds, for their own safety they should never be off lead in public parks. In many parts of the greyhound world it is also illegal to have a greyhound off lead in public.
When greyhounds race and train they do it in very controlled environments, of groomed sand and with muzzled dogs all running in the same direction. Dog parks don't offer the same security.
For the safety of the greyhound, the smaller and lighter dogs and humans present, it is never recommended to run a greyhound in a public dog park.
The risk of injury of a 75 kmh missile running with other unknown dogs and obstacles is just too great. Once the chase starts there is no recall.
If you love your dog, find a private piece of land and have a controlled environment for your greyhound to exercise in.
It is always safety first with your greyhound. So it is always on lead when walking in public but conversely never tie a greyhound up and leave it unattended.