The letter, addressed to Mr Gaudern's mother Ethel, said: "You know we've faced up to the likelihood I may not come back... but you know I feel I shall come back because I want to so much.
"We've had a perfect married life together, haven't we? We must look forward to a more settled future.
"But if I don't come back I want you know how much I owe to you and thank you for our lovely life together and to let you know it isn't my wish that you remain a widow, if you really fall in love again."
The CWGC hopes the archive will be a fitting tribute to the dead and highlight its cemeteries and memorials across the world.
Chief archivist Andrew Fetherston said: "We believe that by capturing these stories from the public we are creating an archive of international importance and a lasting legacy for those who died for our today.
"We want people to share their connections to the war and our cemeteries to ensure that as Commonwealth nations we have not forgotten their sacrifice."