I applied online. I interviewed at Google (New Delhi) in Mar 2018
Interview
First and very important rule, if you want to participate in GSOC, choose some 3-4 organizations from the list of previous participants (Its very probable that they are going to be selected for next year too) and start playing with the tool/application i.e. install, configure, run on local, run for your own test cases etc. The idea is to evaluate your assumed interest for that software and if it turns out to be interesting you would end up being comfortable with it.
After that you need to prepare a proposal and submit it
I applied in-person. I interviewed at Google (Tel Aviv-Yafo) in May 2017
Interview
First I had a phone call. Then I sent my grades sheet. I had 2 interviews. Each one was 45 minutes and after that there was a meeting with the team leader.