Hello, Carol
For F-1 student, a student is allowed to use only up to one year with full-time CPT (>20 hrs/week). But, there is no restriction for part-time CPT (<= 20 hrs/week).
I already use my one year CPT full-time with four internships in the past. I cannot use my CPT full-time no more. Therefore, for GSoC, I want to get work authorization as part-time CPT. When I see the document (as a link below) for CPT, I think I can ask work permit with default = 20 hrs. I will definitely talk with this issue with my mentoring organization. However, during summer, I will work as a full-time during summer,