There's no single GPA that universally defines summa cum laude for a doctorate. Unlike undergraduate degrees where GPA is a common metric, doctoral programs typically don't use GPA in the same way. A doctoral degree is evaluated based on a dissertation, comprehensive exams, publications, and overall scholarly contributions, not a cumulative GPA.
The awarding of "summa cum laude" (or its equivalent like "highest distinction") at the doctoral level is extremely rare, if it exists at all. Doctoral programs usually focus on the quality of the dissertation and overall research contribution, rather than assigning a numerical grade.