Especially in the past in some cultures,Call Name is a legal/formal name in which someone is widely known. Call Name is not related to Given Name so that GRAMPS should not require Call Name to be a part of Given Name. For Example, Dhul-Qarnayn, this is his Call Name, whereas his real name is still debatable.
I suggest... If someone has a Call Name, there should be an option to make the Call Name as the main name to be displayed (instead of the Given Name), and there is an option whether the Given Name wish to be displayed too in the graphical reports or anywhere when it is appropriate.