Sellers lists ten Gillespies in the gun business. Perhaps the likely candidate was George H.D. Gillespie (NY,NY). Importer and hardware dealer. Locks and derringers, mid 1850's. Could have been store-bought locks and anyone could have made a double shotgun with them. Most of the other Gillespies are later and/or not likely candidates, at first glance.
Someone else recently mentioned Birmingham locks. There were hundreds of lock makers in Birmingham in the 19th Century. See Kauffman's The PA-KY Rifle for a good list. He went there himself to compile that list. Wayne