Apply to an organization that could not use your skillset
Everyone is different, and that's a great thing. Only search for organizations that have their source code written in a language you are proficient in, as you will only improve your knowledge in the skill of your choosing for the duration of the GsoC.
Start researching at the last minute
Even though it's the offseason, you have to be prepared for the next edition of GsoC if you want to be chosen. Research your ideas, create a workspace dedicated to this on your machine and keep adding information to it when you stumble upon something relevant.
Start sorting through the organizations that participated last year, as they will most likely also mentor this year's GsoC and figure out which one is the perfect fit for you. That way, you're getting a headstart and learning in the process.
Submit a short application
Your application should showcase all the reasons of why you are applying, why you chose this idea and why you chose this organization. Show them that you truly are passionate about it, provide your research and project plann and some possible future directions.