What is an fgd?
stands for forge/d game data, it has in it all the entities you
need for whatever your working on.
other words, if you are working on a tfc map then you need the
tfc fgd. simple as pie.
you only need one fgd per game / mod for what your doing, so for
counter-strike you wouldn't get the hl.fgd and the cs.fgd you
would just get the cs.fgd one.
You can find all sorts of fgd files on the mods site that you
are working on.
can be found here.
for a good mod site head on over to the Moddb