Applications open today, 20th March, https://developers.google.com/open-source/gsoc/. On April 3 - 24 Organizations review and select student proposals.
@giorgosp However, you might need to be aware that ownCloud distributed system is not operationg on blocks of data, as Dropbox, GDrive and others, but real files are being stored on the server side. This means, there is no actual delta in there. OwnCloud needs to support Object Storages, https://www.druva.com/blog/object-storage-versus-block-storage-understanding-technology-differences/
This implies, that delta here is not out of the box. You cannot just write to Object Storage FILE at some position as in fseek and fwrite. You would need to copy the file from the Storage, apply the delta and store it as [updated file]/[version].
This means, that if you have file of 10MB localy, you need to store it as a single piece on the server, of 10MB. Now lets assume, that you change a bit in your 10MB file, and chunk it into 1MB pieces. You detect that you change only 1-1MB piece, so you transffer your 1MB to the server (yes and this is big win!), but when this PUT arrives on the server, you would need to do 1 storage RTT to fetch a file and copy it, then apply your delta, and have another 10MB RTT to storage to store it (however, this is very fast operation compared to the transfer over the network for modern servers)
If you are still interested in the project, hope the above description helped you to understand what are the requirements. For the application, you need to know definitely both C++ (and maybe some PHP) to develop on server and client. You need to well understand HTTP to easily get around WebDav. You also need to have a well structured understanding how networks work. During your Summer of Code you will work with your mentor (probably me) and as well as all other developers which will help you understand the code base.
I am also strongly encouraging you to submit some patch during application and between 4-30 of May (introduction session with mentor, most probably me with support of other colleagues), we will later announce what patches that could be, so that you get good introduction to the code.