I do not see how to model a gate in sequence diagrams referencing to another diagram.
I am aware of the option 'ref over <>' and 'end ref' where you can include a string inside, but it would be nice to be able to add incoming and outgoing messages to the reference body so as to properly model gates to sub-diagrams according to the UML notation.
In the following link you have an example of such in the ref Balance Lookup:
Is there a way to properly draw this using PlantUML syntax?
It would be be nice to have this feature included if not.
Thanks for your answers.