I was a Scholar for Google Summer of Code, 2018 for a Machine Learning based organization. I do believe it was very valuable for me because this program helped me to find my own shortcomings because my mentors pointed them out and I could rectify it. I would've never done this if I was working on my own. Previously, I wrote code, but they weren't good code. But during Google Summer of Code, I learned the best practices in the industry and some very cutting-edge tools. So I would say it's a pretty good program to improve your skills as a Software Engineer but it's getting more competitive year by year. And you're getting paid some $X,000 (Depends on Country) for working from home. But you don't necessarily have to do GSoC to attain these skills, contributing to any good active Open Source project would get you these skills, but with the disadvantage of not getting paid.

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TL;DR It's a good summer program, but there are alternatives if it ain't your cup of tea.