Interesting GG strategies from TMIT. I don't think I'll be using those, though, because they're addressing a problem I don't usually have. I don't usually go for Domination wins and am only a semi-warmonger (i.e., I never go a game without an aggressive war or two, but intersperse them with long periods of peaceful building). And I usually go for either Space Race or Diplomatic as victory condition.
What that means is that usually when I do open up all-out I have a lot of units to play with, and I'd rather have large numbers of level 3 units than 1 uber-unit and many level 2s. For one thing, I'm seeing more than one highly-promoted defender usually.
I'll have a war in the Ancient Age once in a blue moon when the conditions are right for an axe rush and I don't say, "Screw it, I don't want to axe rush, I'm starting over." I'll have a war (other than just defending myself against some crazy AI attacking) in the Classical Age maybe half the time. When I do, it's often like what happened in my last game, playing Hatshepsut, Isabella hated my guts 'cause I was a filthy heathen, and some friendly AI was at war with her and wanted me to join in. I was sending War Chariots against the barb towns that direction anyway so I said sure, and captured a few of her cities with them. I didn't plan that one, it was a conquest of opportunity purely. Unless I'm playing the Romans, or sometimes an Aggressive leader, I never go all-out for conquest pre-medieval.
So my first really big war is usually in the middle ages, with Maces/Knights/Trebuchets/Pikes/Longbows. I may or may not have gotten one GG before, very rarely more than that. Running Vassalage, I pump out level 3 units from my production cities. I make about 8-10 Trebs. I bring my stack to a city. I use the Trebs to reduce its defenses. I check the odds with my best attacker. If it's less than 50%, I'll attack with a Trebuchet. If not, I'll just go for it. With CRII on the maces, usually I only have to attack with a Treb if the city's on a hill, and I seldom have to use more than one or two. Plus, I have CR II on the Trebs as well, which increases their survival rate. Sure, I lose some maces that way, but not huge numbers, and the rest get nice promotions; I come out of that war with lots of CRIII/Combat I or II maces. Meanwhile, I'm settling GGs in all my top production cities, starting with my Heroic Epic city, so they can continue putting out level 3s after I switch civics.
I'll try to get 3-5 cities with 1 settled GG, and then concentrate on my HE city until it gets up to level 4 units. The goal is to have 3-5 cities putting out level 3s, and the HE city producing level 4s. Usually my medieval war will generate enough GGs to meet the first of those goals, and I have another big one using Rifles/Cavalry/Cannon, continuing with Infantry, which will often give me enough to meet the second goal. Only after that do I start building MAs.
I guess as only a semi-warmonger I don't feel I get enough GGs to use one to make an uber-unit.
I tend to be closer to this approach, really. A peaceful builder who'd rather turtle up and out-tech and/or out-culture everyone else, but who'll maintain a standing army to avoid being harassed. I also tend to only go to war when I believe I can eliminate or permanently vassalize the enemy. I've been trying to be more warmongerly lately, though, simply because I know it's a liability if you don't fight every so often.
At any rate, when it comes to the "one uber unit vs. many level 3 units" issue, I see it as allowing me to have many advanced units later in the game when theocracy and/or vassalization aren't that useful. If you spend the GG to build a level 6 or high level 5 unit, then you can be assured you'll get west point later in the game, which will mean more highly trained units from that particular city. Throw in future GGs as instructors and you're in even better shape.