I believe Perimeter is here due to a deal with Codemasters. Emperor's Testament was published by Paradox Interactive (who owns the GamersGate service, one of GoG's competitors). So it's not quite as straight-forward as one would like to believe.
i bought emperor's testament a few days ago for $5 from gamersgate (i think its still on sale)... but im not sure what it is i bought. the game plays fine by itself, i dont have the original perimeter... how can i tell if emperors testament includes the original or not? if ET comes after the original, i want to play the original first... or does it matter, storywise?
^^ answered my own question buy just going ahead and buying perimeter from gog... yes, emperor's testament from gamersgate is a stand-alone expansion, no it doesnt include anything from the original perimeter, yes it matters from a storyline perspective :)