Police attended an address in the Rhoshirwaun area of Pwllheli shortly before 11.30am on Friday to reports of four people being injured in a dog attack.
Two people were airlifted to hospital from the property, while two others suffered minor injuries, North Wales Police said.
The dog involved has been put down and is being examined by a vet to establish the breed.
Another 37 dogs and a number of cats have been seized from the address, the force said.
Chief Superintendent Sian Beck said: “We understand this was a concerning incident in the local area and wish to reassure the community that there is no further risk to the wider public.
“We have launched a joint investigation with the RSPCA and are appealing for anyone who may have information about the dogs located at the property to contact us.
“We would like to thank members of the local community for their patience and support whilst we were in attendance at this incident.”
A police presence remains in the area while inquiries continue.
North Wales Police have urged anyone with information on the incident to contact them by calling 101 and referencing A182569.
In October, a school was forced to evacuate in Norfolk after a 60-year-old woman was mauled by her pet XL Bully at her home, before it escaped from the back garden.
Rishi Sunak has announced the dogs will be banned by the end of the year following a spate of horrific attacks.
The prime minister said the animals, which campaigners have linked to at least 14 human deaths since 2021, are a danger to children and communities, in a video posted to social media.