The George Gibbs firm is now owned by another company called I M Crudgington. They are still in existence and could give you details if you want them. Try contacting them through this website. They are a forum of the British gun trade and could help. I think Griffin and Howe and Hoffman Arms also made some guns in the US.
Is this question about a specific gun that you're planning to buy or is it a general one?
Best wishes and good hunting!
-------------------- The Ark was made by amateurs. Experts built the Titanic.