Headers and code needed for a C++ hack?

I tried GLua Headers, but I got stuff like how "pLuaShared" wasn't a variable and stuff.

if it's not a variable it's probably not a variable

make it a variable

i dont know what type it should be though
and its more than just that i get errors with

You don't *need* the source sdk, but you can use it. Or you could just make your own little SDK that only has the stuff you need in it, which can be found with a little bit of reversing in olly/ida (or look at someone elses, and test/change it to work on garry's mod).
There are many open source cheats available with just a little bit of searching. I haven't really looked at many of these but https://github.com/skimmilk/skimbot looks like a good source (made for TF2) that you could use as a reference.

yes use things made 5 year ago +rep

like garry even dev's anymore Xd