You don't qualify what you mean by "in-depth" but keep in mind this topic is suitable for PhD-level research. There are not many people out there who have the necessary background and would attempt to implement a GPU-accelerated transcoder for free.
This also happens to be an area of active research. For anyone interested:
1. Motion estimation for H.264/AVC on multiple GPUs using NVIDIA CUDA
2. Intra Frame Encoding Using Programmable Graphics Hardware
3. H.264 Video Encoding Algorithm on Cell Broadband Engine