Hi Théo,
Welcome! Pleasure to meet you!
As a former GSoC student myself, I advise you to look for something you fancy instead of just looking for something that fits your skill set. It's way better to find something you like (even if you're not 100% familiar with it) instead of looking for something that perfectly fits your existing skill set; remember, you're also here to learn :)
Of course, the tasks are just examples and you can diverge from those if you'd like.
For a brief overview of the project:
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https://github.com/tianocore/edk2 is the main EDK2 repo that contains lots of support code (that's used throughout most/all platforms) and OVMF (edk2-on-virtual machines, like QEMU; good for testing your changes). Most of the development is done here. The build tools also reside here. You can look at the various packages inside edk2 to see if there's anything you like.
Of course, you also have more repos like edk2-test, edk2-staging, etc :)
So yeah, take a look and see if there's anything you like :) Of course, if you could tell us more about yourself and your interests inside embedded programming, it would be easier to help out.
Best regards,
Pedro