There is a “How do I…” category wiki article on “Add a Witness”.
Event roles and Association roles are two explicit ways of creating relationships in Gramps. (The Family container is another. It has Implicit roles connecting spouses to each other and their offspring. Another explicit but often overlooked method is the Internal link feature in Notes.)
Sharing Julia & Peter’s marriage Event (which is a ‘Family’ role in their Family container) to be an Event with a ‘Witness’ role for ‘John’ is the best approach. It ties a lot more dimensions and proof into how Julia & Peter relate to John than using Associations.
Each of the Associations is unidirectional (not bidirection nor omnidirectional) and fairly one dimensional. Although it allows a little breadth with the ability to add sources & notes, it only specifies that Person1 has a relationship to Person2. There isn’t a relationship in the other direction. (“Sheldon” might have a ‘fanboy’ Association role with “Albert Einstein” but Einstein has no reciprocal relationship to Sheldon.)