I'm currently playing a strength based variant human spell-less ranger. The party is level six right now, and I'm am far and away the hardest hitting party member, though I do not expect to gain more combat power for the next several levels. For reference, the party consists of a gnome wizard (necromancer), V human dual wielding champion, and a V human dex paladin.
We used point buy for stats, so everyone is on the same level. My feats were mounted combatant and the arguably broken polearm master, and it has been devastating. I am the most mobile party member by a long shot since I can ride far each turn and my glaive keeps me out of reach most of the time. With my haft attack bonus action and Horde breaker, I am making four attacks on my turn almost every round, and I generally also get another from enemies running at me. Putting Menacing Attack on that AoO generally prevents them getting close enough to hit me, letting me freely ride away next turn. Oh, and don't forget that I've got advantage on any medium or smaller targets while mounted. Next level I will impose disadvantage on opportunity attacks, letting me skirmish even more effectively. When my DM gave me a belt of stone giant strength (23 str) things started bursting at the seams.
Now, a lot of that comes from 1) being a spell-less ranger, 2) having feats, and 3) using hit and run tactics. From now on I will not be gaining any more offensive abilities other than proficiency bonus increases, and I'm fine with that. While mounted I almost never get hit, almost never miss, and can control enemy movements with ease. I am also the best at wilderness exploration. I track enemies with ease, scout ahead to prevent ambushes, and provide healing support. Sure, a fighter might hit things a tad harder, but that is worth nothing when you can't find your targets in the first place.
Right now, as a sixth level ranger and subtracting my belt, I deal 3x(1d10+3) + 1d4+3 = 31 points of damage every turn. Add 8 if someone approaches me. Add up to 18 for maneuvers. Using your assessment above (5 rounds combat, no misses, no crits), my level six ranger deals 213 points of damage.
Since you mention GWM and in the interest of a fair contest, I will consider my ranger at level 19, no magic items, and with the GWM feat. That's 3x(1d10+15) + 1d4+15 = 79, with an extra 20 when approached, and 27 from maneuvers. I get a final count of 522 after five rounds. Keep in mind that I'll have striking with advantage to offset the -5 from GWM if I am mounted and facing medium or smaller foes. I will also be dancing away from enemies each turn with impunity. AND I will still be better than any fighter at navigating in the woods, knowing the weaknesses of my enemies, and setting up ambushes.
Lastly, suppose my ranger was a spellcaster instead. I'd still deal 99 damage per turn, however I would have an additional 4d6 = 14 every round from hunter's mark. That's 565 damage.
Alternatively, I could cast Spike Growth on the first turn, grapple someone the second, and have my horse ride 120 feet around the outside every turn for 48d4 = 120 damage each round. Dealing 1d8+5 on a hit, that's seven attacks for a total of 66 + 480 = 546 from dragging their face through spikes. I'm also ignoring a potential 45 damage from Horde Breaker.
By your metric, they are certainly behind other martial classes in pure damage output. Of course, damage output is only a slender fraction of the entire picture. Being able to avoid retaliatory hits is spectacular. Who would you rather be? The fighter that deals tons of damage but gets swarmed and eaten by kobolds, or the ranger who cuts through the enemy nearly as effectively and then lives to fight another day?
Damage per round means less than nothing in the end. There are too many factors that are impossible to quantify.