If you're doing a marathon, do Marathon
. No it's not a GOG, yet. It's free though (and open-source no less).
That's what I was about to say...and if you're still craving more, you can go through Marathon: EVIL, and Marathon: Tempus Irae, two total-conversion mods which are meaty enough to be full stand-alone games in the own right.
And you should also be sure to play the current king of Marathon TC mods, the unofficial "Marathon 4", Marathon Rubicon.