Thank you for your inquiry. The short answer is yes, being at Treasure Island counted as being in the Asiatic-Pacific Theater. However, I would state it somewhat differently: Armed Guard personnel in the Asiatic-Pacific Theater were more than likely assigned to the Treasure Island Armed Guard Center.
In greater detail, there were three Armed Guard Centers during the war, as you are probably aware: Brooklyn, New Orleans and Treasure Island (in San Francisco Bay). However, the Armed Guard Centers did not really function as long-term duty stations. Armed Guard personnel were formally assigned to one of the Centers, where their personnel records were maintained. Armed Guard crews typically were assigned to a given vessel for several months or several voyages. Then they were rotated back to their assigned Armed Guard Center for R&R, living at their Center until they were randomly assigned to their next ship, a period of likely several weeks. Armed Guard Centers had barracks, a mess hall, a chapel, medical and dental facilities, recreational facilities, etc. While the Centers were the home away from home for Armed Guard personnel between shipboard assignments, the men who were at a Center at any given time did not perform naval functions other than simply living there temporarily. The Brooklyn Center assigned men to ships heading for Atlantic and Mediterranean destinations, the New Orleans Center concentrated on destinations in the Gulf of Mexico, Caribbean, and South Atlantic, and the Treasure Island Center had the responsibility for Pacific and Indian Ocean destinations. Thus, with rare exceptions, Armed Guard personnel in the Pacific theater were more than likely assigned to the Treasure Island Center, and vice versa, if assigned to the Treasure Island Center, they generally found themselves serving in the Pacific.
It is apparent that you or your family have at least some of your great-grandfather’s records but you may not have his entire military service record. If this is the case, you may wish to request a copy of his complete official military service record. Please see this page from the Armed Guard website that I manage: http://armed-guard.com/searchmil.html. In particular see section A.1. Records of Individuals - U.S. Military. You will have to contact the National Personnel Records Center in St. Louis, MO, a facility operated by the U.S. Archives. Provide as much identifying information as possible about your great-grandfather. The links on the web page above will take you to the necessary pages of the Records Center web site. There will be a fee for researching, photocopying and mailing the information but the Records Center staff will not begin research without informing you of any charge. His military service record would contain such information as dates of enlistment and discharge, the ships to which he was assigned, applicable dates, training, any decorations or medals earned, etc.
Note that someone such as yourself, who is not next of kin to your great-grandfather, may not be able to obtain his complete service record (next of kin = parent, spouse, sibling, child). Disciplinary or medical records might not be available to you, for example. If there is someone in your family still living who is next of kin to your great-grandfather, such as a grandparent or a great-aunt or -uncle, it would be best if the request to the Records Center was submitted by or in the name of that person. You can do the legwork of research, completing forms and otherwise preparing the request but the request should be signed by that person. If there is no longer a person who is next of kin still living, then make the request yourself and hope for the best. There should be something of interest to you even if not his full record.
Good luck and best wishes.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website