Gunnersbury Large Mansion is now the Gunnersbury Park Museum, which contains a collection of local histroy and archaeology. Like the Small Mansion, the state of the maintenance of the building is appalling. On the left side of the Large Mansion the Arch is visible.
Gunnersbury Park is a park in the Brentford ward of the London Borough of Hounslow, in west London. Purchased for the nation from the Rothschild family, it was opened to the public by Neville Chamberlain, then Minister of Health, on 21 May 1926. The park is currently jointly managed by Ealing and Hounslow borough councils. See here