I am going to need more details... because I have done a lot of math for a lot of possible situations... and that just doesn't happen. Maybe if you manage to always have advantage against an AC 12 creature..... but otherwise those numbers just don't make sense.
Our party used the following. Bard would cast
bless. Action Surging battlemaster would sit on his Battlemaster Dice to use precision for those attacks he missed.
Let's say attack sequences looks something like this.
1. Wizard casts
fly on GWM fighter.
2. Fighter flies in to dragon. Uses Action Surge. He has a +1 sword and is 11th level. Adult Dragon AC is 18. He gets +4 prof. +4 str. +1 sword +1d4 with
bless.
Hit roll: +11 without GWM Dam: 2d6+5 for 12 average damage.
Hit roll: +6 with GWM. Dam: 2d6+15 for 22 average damage
Against an AC of 18.
Average percentage to hit without GWM: 65%
Average percentage to hit with GWM: 40%
So average damage against AC 18 without GWM with 3 attacks per round equaling two hits: 24 points
So average damage against AC 18 with GWM with 3 attacks per round equaling 1 hit: 22 points
Now incorporate the nova attack blowing Battlemaster Dice adding 1d8 to each attack roll that misses and having a bardic inspiration dice as well, an average bonus to hit 4.5.
Damage without GWM: 36 points
Damage with GWM three hits: 66 points.
Not quite double, but around 80% damage boosted during a nova with battle master dice. Action Surge. You're looking at the difference between 72 points and 132 points. Then there is the variable of the d4 from
bless and general variable die rolls on the d20. If the AC is lower, it becomes even easier to nova such as it was against the non-dragon caster and prophet we fought.
The smiting paladin is more theory-crafting since he did not have GWM. A 3rd level spell with greater divine smite does 5d8 smite damage. With longsword that is 2d6+5 plus 5d8 for an average 39 point hit. GWM adds +10 damage. It's around a possible 26% increase in damage for a smiting paladin. Not quite as bad as the fighter. Given the paladin has two vows that completely eliminate the -5 penalty, he probably hits more often than the fighter in single target situations.
So it's looking closer to an 80% increase for the Action Surging Battle Master fighter (Champion won't see the same results due to no means to boost hit roll and GWM not being effected by crits). Eldritch Knight may if invisible or using some other means to boost attack.
Paladin will see a roughly 26% increase in damage all things being equal.
Barbarian can probably use GWM better than either of the above classes. The Battlemaster fighter took a couple of levels of barbarian later on, he was using GWM with Reckless all the time. Maybe did not use a few times when fighting someone with an AC in the 20 range, but still novaed with GWM and Reckless for a lot of damage.
Sharpshooter is closer to doubling damage and easier to use due to bonus to hit with Archery style as well as eliminating pesky problems like cover with lower damage bow. Average bow damage is 1d8+4. Sharpshooter doubles that.
The problem is narrow from my perspective. I don't care if the martial uses GWM or Sharpshooter against multiple targets. Clearing trash fast is fine as I don't like spending a long time rolling for multiple combatants. It's those instances where the party is novaing on a single target that really chaps my hide with the Sharpshooter and GWM unloading with both feats using multiple attacks. Maybe non-minmaxers or those that don't track damage don't notice the difference, but for those of us that do track this stuff it is a very annoying effect. Makes playing enemy casters and single target monsters not particularly fun as a DM. When a big dragon has to run from a Sharpshooter bowmen or GWM fighter backed up by a party because they will output far more damage than he will, that isn't a fun bit of action imagery in my mind when DMing.
"Look at that, the dragon tucked his tail and ran from the bowman and fighter. They hit him way too hard. Poor little (I mean huge) dragon."
Or the even worse:
Your players saying, "That was the main encounter? That guy? The guy in the magic mask? He was supposed to be a tough fight? Oh." This is followed by a shrug, while the DM wonders why the guy was so weak.