Even “Stone Frigates” needed a crew to run them [in this case at least boilers for heating together with filtration equipment for the swimming pool] and to assist the Training Teams with evolutions [practice operations] associated with machinery [e.g. pumps for the damage control school] or boilers, so he was probably part of the “permanent staff”. Being 50 years old at the time, his presence would have released a younger man for service at sea or ashore in France with Naval Brigade.
The notation 'Stoker RN HMS Ganges' (Stoneman in Coalmine), suggests he was either a former Royal Navy seaman who had left the RN once, taken up civilian employment and been recalled from the Fleet Reserve, or volunteered for war service and that would indicate he was actually a member of the Royal Naval Volunteer Reserve [RNVR] rather than the RN. As his occupation was not connected with shipping or fishing it’s unlikely he was member of the Royal Naval Reserve [RNR].
Records for Royal Naval Ratings from 1853 to 1923 and Royal Naval Volunteer Reserve [WWI] are available online from http://www.nationalarchives.gov.uk/documentsonline/navy.asp
The only records held for HMS Ganges at the National Archives, Kew covering the WWI years are in ADM 338 Admiralty and Ministry of Defence: Chaplain of the Fleet and successors: Registers of Baptisms, Confirmations, Marriages and Burials.
The Royal Navy’s Historical Branch may have some operational information but they will not have any records for personnel.
There’s plenty of information about HMS Ganges on the web as well as forums etc.