After reading Applying UML and Patterns (Larman) and looking around on the Internet it still seems ambiguous to me if a UML communication diagram should depict implementation details (such as GUI related function calls) or be more bound to the analysis. What is the correct answer? Does UML even specify it?
Well, it depends on the view the diagram belongs too.
If the diagram is part of the Logical View or the Use Case View, it should not dive into the very details of the implementation because this view is concerned with the functionality provided to end users and may be used communicate the architecture to end users, domain experts and others who do not care much about the internal functions.
On the other hand if the diagram is part of the Development View, it's your call. The development view is used to communicate the architecture to the developers. So if you believe the details of GUI function calls and so on will help the developer understand what you want then it's Ok.