Thanks Bob and Kent for the answers, that pretty much solved the issue. In this scenario, just changing
xmlns:CustomDepen="clr-namespace:ImageGUI.App_Code;assembly=ImageGUI"
to
xmlns:CustomDepen="clr-namespace:ImageGUI.App_Code"
fixed the situation. Everything else was correct.
Regarding my other comment about how to retrieve the specified value, it would be like that:
EAcessLevel currentAcess = (EAcessLevel)gbApplications.GetValue(DependencyPropertiesHoster.AcessLevelProperty);
Thanks, and hope it also helps somebody in the future.