Hi Tane,
great to hear that you want to contribute to GSoC and GNURadio. Bad things first (well not really too bad ;) ), you should have some knowledge about the operations used in forward error correction. There are several techniques used (LDPC, Turbo-Decoder, Viterbi) but you don't have to know all of them. It is also a plus if you know in advance what kind of channel code you want to implement as it affects the way it can be optimized. If you don't know anything about decoders, a strong knowledge in optimizing, especially vectorizing, algorithms might help equally.
If you want some good places to start I would recommend the "GNURadio Guided Tutorials" [0], the documentation on VOLK [1] and the FEC-API [2] (the GNURadio Forward Error Correction Framework).