take the existing hl2 fgd and modify it
#how can i create a .fgd for a Hammer Config?
misty raven
you can pretty easily study it and see how it works, its pretty simple
since you're making a source sdk 2013 mod you can just start with that fgd and modify it as needed
open it in a text editor
there is documentation here for how things work and what they are, but it is also really easy to figure out by looking at it