Merging codebases is almost always something that's done when a single codebase is forked, then those forks get merged back into eachother for improved functionality/performance.
The problem here is that DeKP is definitely not a fork of the GRSS so the best thing I can do for you here is ask: what functionality are you looking to have added to the GRSS that DeKP doesn't have?
I can answer questions about the GRSS mod and some about WoW modding in general, but your request in its current form is not trivial at all. That is, it's not something someone can "just do real quick."
If you're looking to get into WoW modding, I strongly recommend picking up the book
Programming in Lua, written by the guy that invented the Lua language. I used this book to learn the language, and I still regularly reference this book when I'm working on the mods.