When a Muggle touched it, it would emmit a Muggle-repelling alarm of the user's choice
The Muggle-Deterring Gate was an invention advertised by the Rumours! magazine. Whenever a Muggle would touch the gate, it would emmit a Muggle-repelling alarm of the user's choice. The choices included a ghostly wail of woe, a sinister cackle of laughter, or an eerie scream of pain. The product was not approved by Ministry of Magic.
- J.K.Rowling Official Site (Mentioned only)