I really am okay with the GDR. I can't see myself building very many, ever, but hopefully the AI will, and then I'll actually have a fight on my hands?
I think the GDR works in Civ VI overall. Civ is kind of Science Fiction the same way the Neil Stephenson's Baroque Trilogy is Science Fiction. Caveman to the stars stuff. Plausibility wise, the biggest problem with the GDR is going to be the Square Cube Law, which means its cute adorable legs and skeleton won't be able to support its weight, let alone jump mountains. But, sure, build it with nano layered hand-wavium composite materials and it'll be fine.
The real problem with the (enjoyable) silliness of the GDR is that it lacks context. Where are the other small sci-fi things building up to that, which would make it seem more plausible within Civ VI's universe? I mean, you'd think there would be smaller mech units or drones first, right? Or more advance power systems (e.g. cold fusion)? Nope. We just go from conventional guns and tanks to CRAZY GIANT ROBOT MECHA KILLING MACHINE THINGY!!! ARGHHH!!!!
I mean, I know none of the Future Tech stuff is going to matter, because the game will be long over before then, but it's all looking a bit bare in the future particularly with no future Great People. If we're going to have future stuff, can we actually have, you know, future stuff?
I'm not saying Civ should go all Gonzo in the Future Era, and start pulling in lots of crazy stuff. Man, I really don't want phasers and teleporting and all the Star Trek Star Wards Flash Gordon rubbish. Instead, flesh out the future a bit with more plausible real world but speculative sci-fi stuff (not just military, but other stuff) and then build up to the crazy GDR.
BTW, why don't you need a special district to build a GDR? Or at least, you know, a Factory and or a Research Lab? You need a special district for Bomber-planes, but not for Giant Pacific Rim Jaegers???
I like the GDR, but FXS just aren't really... "selling" it, I guess.