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Officers went to an address in the Rhoshirwaun area, near Pwllheli, just before 11.30am on Friday.
Two other people suffered minor injuries.
The dog has been destroyed, but the breed has not yet been established. A vet will be asked to examine it.
Another 37 dogs and a number of cats have been seized. The RSPCA is working with North Wales Police.
Officers are maintaining a presence in the area and inquiries are continuing, the force said.
However, Chief Superintendent Sian Beck said there was "no further risk to the wider public".
She added: "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."