What does the term "marked for encryption" mean in the context of public key encryption?

We're trying to troubleshoot a situation where some files sent to us can be successfully decrypted on the command line using GPG but not in a GUI program that interfaces with that library. The error reported by the GUI program is "the key is not marked for encryption". What does it mean for a key to be "marked" for encryption? When is that marking done? When the key is first created, or later, when the key is used to encrypt a file?