A 63-year-old man, believed to be the shop’s owner, Bob Gallichan, was arrested on suspicion of arson following the blaze.
The suspect was found dead on the day he was due to answer bail.
Last week an inquest was opened into the death of Mr Gallichan (63), who was found hanged at his St Saviour home.
The States police have declined to confirm whether Mr Gallichan was the suspect.
A spokeswoman for the States police said that officers were not looking for anyone else in connection with the incident.
She added that the investigation was continuing but was expected to be complete by mid-March.
At least 20 Islanders fled their homes when the fire ripped through Gallichan Marine at about 5 am on Monday 7 December.
The Harbour Gallery, which is joined to the back of Gallichan Marine, suffered fire, smoke and water damage.