I only today discovered your message but would like to respond to it, however belatedly.
From what I have found, SS CHRISTOPHER GADSDEN was constructed by the North Carolina Shipbuilding Company in Wilmington, North Carolina, in 46 days between November 15 and December 31, 1942. She survived the war and was scrapped in either 1969 or 1970; two sources differ on the year she was scrapped. See http://www.shipbuildinghistory.com/history/shipyards/4emergencylarge/wwtwo/northcarolina.htm and scroll to hull number 50. Also see www.mariners-l.co.uk/LibShipsC.html and scroll to the name of the ship. This rules out the loss of the ship in March 1943 and the matter of a lone survivor.
Another web page, listing ships sunk or damaged in chronological order during World War II, shows that no ships were lost on 24 March 1943 or for several days before or after. See www.usmm.org/sunk43.html. Interestingly, CHRISTOPHER GADSDEN appears on that same list on 30 March 1943. However the type of damage or loss is noted as “Unknown” and there were no crew deaths listed, although one Armed Guard sailor was injured.
From yet another source I found that CHRISTOPHER GADSDEN was in convoy TE-19X between Gibraltar and Philippeville (now Skikda), Algeria, between March 16 and March 20, 1943. She apparently remained in Philippeville until March 28, then departed in convoy ET-16, arriving in Oran, Algeria, on March 31. These convoy operations and intervening port visit include the dates of March 24 and March 30, the first date being the one you note as the date the ship was sunk and the second being the date on which something may have happened aboard the ship per www.usmm.org/sunk43.html. However the records of the two convoys contain no indication that anything unusual befell the ship on either date. See http://convoyweb.org.uk/ports/index.html?search.php?vessel=CHRISTOPHER%20GADSDEN~armain; detailed information for the two convoys are linked from this page. The ship eventually returned to the United States in convoy GUS-6, departing Oran on April 10 and arriving in Hampton Roads (i.e., Norfolk, Virginia) on April 28, 1943.
In short I can find no evidence to confirm your statements about the loss of CHRISTOPHER GADSDEN while your grandfather was aboard, nor that he was the sole survivor of a sinking. To be sure even original records can contain errors, but whether a given ship sank on or about a given date seems not to be a matter that is easily disputed when independent sources indicate the ship continued to sail long after that date. So a question might be whether there was an incident involving your grandfather aboard a different ship and/or on a different date.
If you wish to pursue this further, you may be able to obtain a copy of your grandfather’s military service record which should indicate the ships to which he was assigned and the applicable dates. Please see this page from the Armed Guard website: 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 grandfather. The links on this web page will take you to the necessary pages of the Records Center web site. There may be a fee for obtaining the information but the Records Center staff will not begin research without informing you of any charges. In addition to the ships to which he was assigned and corresponding dates, your grandfather's military service record may contain such information as enlistment date, discharge date, training, illnesses or injuries (which might point to a shipwreck experience or other onboard event), any decorations or medals earned, etc.
Note that someone such as yourself, who is not next of kin to your grandfather, may not be able to obtain his full service record. (Next of kin = parent, spouse, sibling, child.) If there is someone still living in your family who is next of kin to your grandfather, presumably your grandmother, mother/father or an aunt/uncle, it would be best if the request to the Records Center was submitted by 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.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website