Gunnersbury is wholly within the borough of Hounslow, so that is the organisation which manages the Park’s daily activities.
Hounslow’s Civic Centre general enquiries phone number is 020 8583 2000. It is staffed from 9 am.
But if you are phoning during the week after 5 pm, or after 4.45 on Fridays and over the weekend, you will need the “out of hours emergency contact” number: 020 8583 2222.
The service which looks after Park security is Parkguard Ltd. They don’t seem to have a helpline so you would need to inform the Civic Centre, but they do have a twitter account: @ParkguardLtd
The Gunnersbury Park main office is 020 8993 8809.