gpg: xxxx: skipped: public key not found

Question

I'm getting this error:

gpg: xxxx: skipped: public key not found gpg: [stdin]: encryption failed: public key not found

I'm using this code:

IEncryptionService encryptionService = new EncryptionService(@"C:\GNU\GnuPG\gpg.exe"); 
var encryptedFile = encryptionService.EncryptFile("xxxx", @"D:\application\" + NewFileName + ".GFM", @"D:\GPGFILE\" + NewFileName + ".GPG"); 

I would appreciate any guidance, advice, or assistance in helping me get this resolved.