The GDR is just there to prevent domination games from ending in a slogfest.
Many players on this forum do not play this way, but two large end game empires geared for production can absolutely drag things out. When you have numerous cities that can put out a modern armor army in 6 turns, huge military budgets, and ridiculous city defense numbers, it is almost impossible without nukes to actually beat down a peer civ, especially if neither has a leg up (like if you both take fascism.) It takes a lot of bombing runs to dent cities with 120 defense.
Some RTS games, like the Age of Empires series, have incorporated late game upgrades that are absolutely OP, precisely for the reason of giving the side that gets them a decisive advantage to break a stalemate.
My only beef with GDRs as shown is that they are too good- the only counter is another GDR. Why do I dislike this? Because then the late game is a uranium hoarding race.
The combat system is set up so that 30 strength difference is the threshold for instant kill. So any land unit that isn't over 100 strength (only modern armor and mech inf armies match that) will die in one hit
if they attack the robot. (so you can hardly go on the offensive if they bring a supply convoy.) So if Korea gets GDRs first, even a huge military power with tons of oil reserves can't take that thing down. (And the GDR player has uranium, so they could just nuke your conventional military away in one strike.)
Currently, a GDR costs about what a modern armor army costs and it has a 23 strength advantage. Unless you have a ~3:1 production and gold advantage (GDRs cost 15 gold maint, Mod Armor army costs 16) you are going to get steamrolled so long as they have uranium. The anti air upgrade may doom air attacks which would otherwise be your best bet if you can pick up the vs land units upgrades. On the ground, shaka, scythia, and the aztecs are the best bets because they can all get extra combat strength to whittle down that GDR advantage to something where it can actually be fought. Late game aztecs probably take the cake here if they have enough luxuries. Conversely, Aztec GDRs may be impossible to stop.
As can be seen, Aztec myth of Cipactli, the Sea monster, clearly depicts a GDR walking through the water and wrecking everyone on a luxury fueled rampage.
I think the spikes are Anti-aircraft guns.