Does previous participation in Google Summer of Code help when applying for a Master's program?
Applying to Masters Program does not really depend on one single factor but a variety of factors. 

The above sentence is very important before we move on to the answer. The decision on your admit depends on your application as a package and will be evaluated in its entirety. If your are a local student and then you attend Google Summer Of Code as part of the general trend of your location, then its obvious that most of the people from your school would have done so as well. In that case, what factors in is your school an its culture. 

But say, if your are an international student and from a non-reputed institution, then the grad schools does not really know a lot about your institution, but then what matters is how you have excelled in your institution. Are you an student, who just want to finish of your course work or wanting to do beyond something? Here what you have done matters a lot.

Now the school where you are applying counts in as well. Some programs in a school would need a certain kind of attitude to survive. So it all boils down to how you project your resume, SOP (statement of Purpose) and how can good you have come up with what you have?

So most of grad schools know students dont have the luxury of doing all things needed for a program, but will seek to know how the applicant has fared in his life. 

So it does matter a lot unless you did study in school where just most of them would have done something beyond just academics or apply to a program where you expect to see a lot of these performers apply too. 

Just highlight them in way that is appealing to the school. Like 'I am 2 of 100 people in my school who participated in Goodle Summer of Code'. Strike it like its still hot. 

Source: Personal Experience/Opinion when applying.
I did focus on highlighting things I did and my other folks in college did not..