Two men have been questioned after cannabis was found at Plas Glynllifon near Llandwrog.
A police cordon was set up around the 19th century mansion in April, while detectives carried out a search under the Misuse of Drugs Act.
North Wales Police said a 70-year-old man was arrested on suspicion of allowing the property to be used for the supply of cannabis - he has since been released on bail.
A 38 year-old man, arrested on suspicion of supplying cannabis, has been released but remains under investigation.
The mansion has remained empty since falling into receivership in 2020, when plans to turn the building into a hotel failed.
It was brought in 2022 by Manchester-based businessman Davis Savage, but put back up for sale just two years later.
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