I'm currently a cs masters student and write my thesis this summer (dont have a project yet). I wonder if I will be able to find a project in GSoC and write a thesis with a mentor ?

the focus of GSOC is code, so your thesis would have to be very code heavy for that to work. do you have a topic yet? if not, look at what organizations/projects fit your field of study/interest, and talk to your supervisors if you could write a thesis around one of those.

then (if accepted) you work on the code for GSOC and write your thesis for your school.

Thanks this helps. Yes I want to combine software development with some sort of machine learning. I think this applies to almost every analytical field. In GSoC Orgs like learning software for kids are interesting. But I have to dive deeper into the projects offered..