At this stage of the program you should reach out to the organization using the preferred contact methods listed on their page on the GSoC site. If there is a Contributor Guidance link on their page they will have very specific instructions on how they want you to contact their org. Most orgs have a chat channel or a mailing list they want people to reach out to.
Remember right now each org is receiving dozens, and even hundreds, of inquiries from potential GSoC contributors so they have a preferred process in place of how they want to manage all of the incoming questions.
You should not be trying to contact a particular mentor at this point in the program unless the org's instructions specifically direct you to a mentor's personal contact information for a given project idea (which is rare).
If people reached out to mentors directly at this point they would receive significant spam and that is not okay.
- GSoC Admins