I've been reading forum posts, reddit comments and trade chat, and the general consensus is that Gladiator DPS is good enough, and you can play whatever you want until you get to mythic. Alternatively, if you spec it you're a scrub, get out of the raid, you can't DPS as tank.
Both are wrong.
First off, a Warrior in Gladiator Stance is not a tank. While some of the survivability of Prot is retained, it isn't significantly greater than Fury or Arms. Shield Charge replaces Shield Block, and Shield Barrier is very weak without Resolve, so Prot's active mitigation is absent in Gladiator Stance. Gladiator does still have Demoralizing Shout, Shield Wall and Last Stand (as well as much higher armor and the ability to block), so they're able to take some hits, but not for long.
So Gladiator is not a proper tank, but is it a proper DPS? Let's see what the sims say:
At 685 ilevel, Glad is 98.6% of median DPS, 94.3% of Fury 2h, above Afflic Warlocks, Assassin Rogues, Frost DKs, Arms Warriors, WW Monks and AS and VE Shadow Priests, and slightly below Destro and Demo Warlocks.
At 700 ilevel, Glad is 98.6% of median DPS, 92.5% of Fury 2h, above both 1h and 2h WW Monk, Enhance Shaman, AS and VE Shadow Priest, and barely below Frost 2h.
This is what 1.4% from the median looks like.
The DPS spread is 89.7%-116.5% in normal BRF, 89.2%-116.8% in heroic BRF and 88.3%-116.6% in mythic BRF. At each level, Gladiator is only 1.4% lower than median DPS, so to say that Gladiator is not viable is to effectively demand that everyone be at or above median DPS. At all levels, this excludes WW Monks, Shadow Priests that need to move, Enhance Shaman and Frost DKs.
Of course only one fight in BRF is anywhere near Patchwerk, so single target sims are only one side of the story. Frost and Enhance have killer burst AoE, Shadow and Enhance have great sustained AoE, and WW has good cleave DPS. Gladiator has average single target DPS and if these two experimental sims (4 target sustained AoE / Patchwerk+15sec 8 adds every 45sec) are anywhere near accurate, it has average burst and average sustained DPS
So Gladiator isn't bad at single target, cleave or AoE. It's just average. Not "obsolete" or "not viable", just average. Why, then, play an average spec when Fury is 5.7%-7.5% better?
Because it's sexy.
Well, in my case, I leveled and started raiding as Prot, so I have bonus armor on my neck, cloak, rings and trinkets and I have a good 1h and shield. I need 7-8 pieces of 640-655+ gear before I can play Fury or Arms. Additionally, Gladiator is actually better than Fury below BRF ilevels (Arms is better too). Fury needs a lot of crit to play well.
"Why not just play as Prot?", some might ask. Well, on my first pug into HM, I was asked if I have a DPS offspec. This raid leader was OK with inviting a 640 ilevel tank, but after getting a higher geared tank, they asked me to switch. I think this is a reasonable request to make, so I switched to Gladiator. (I was promptly kicked, of course, which is unreasonable.) This is the primary niche that Gladiator Stance fills, allowing tanks to switch to DPS without already having a separate set of gear.
Going though HM and BRF LFR as a Gladiator, I've actually been ranking high on the meters. At 640 ilevel with Hellscream's Doomblade (About equal to Beakbreaker Scimitar), I was doing 15-17k DPS. With a few upgrades and some practice, I got over 20k. Now that I've crafted a Steelforged Saber and gotten Absalom's Bloody Bulwark, I expect to improve even more. On the one hand, I'm very new to this class, but on the other, I'm well practiced at climbing the skill curve of learning a new rotation. Either way, I think it's reasonable to conclude from my own experience so far that Gladiator DPS is not just viable, but good at this level.
Even at the BRF ilevels, it's probably easier for a geared tank to temporarily switch to Gladiator than to build a separate Fury set. Conversely, it's easier for a Gladiator to fill in as tank. The stat priorities for Prot and Gladiator are different (mastery is high for Prot, low for Gladiator), but the basic tanking tools (bonus armor pieces, 1h and shield) are already present.
Those are all temporary examples, though. What if a player intends to optimize for Gladiator through, say, Heroic BRF progression? There's three major issues to address:
First, Gladiator lacks utility, specifically Rallying Cry. This may not be a big deal in normal mode, but in heroics and certainly mythics, this can mean sitting out for a fight with a particularly difficult phase. Also, Gladiator has few DPS cooldowns. Talents give Dragon Roar and either Bloodbath (single target) or Bladestorm (AoE). That's it. Having an execute doesn't actually help much either, since mixing it into the rotation takes a lot of skill for only a small gain and messing it up decreases DPS. (Survival Hunters are in a similar situation, having only one cooldown from talents with Stampede, and no execute, so Gladiator isn't alone in having little burst/execute DPS.)
Second, Gladiator uses tank gear, so in a raid with 2 shield tanks, Gladiators will be 3rd in line every time for 8 slots of gear. This is a mixed blessing, however. Other classes might be lucky with their rolls and/or DKP and get their upgrade 1st, or unlucky and get their upgrades 4th, 5th or even 6th. Gladiators will always get theirs 3rd.
Ultimately, both of these concerns depend on a particular group composition. If a given raid team has enough raid cooldowns from other DPS Warriors bringing Rallying Cry, Smoke Bomb from Rogues or Amplify Magic from Mages, ect., then not having an additional Rallying Cry is less important. If only 1 or neither tank uses shields, then the gearing conflict is less important.
The last major issue, though, is that Gladiator is only doing average DPS, while Fury consistently does above average. Ultimately, this is the sticking point. A progression minded Warrior should choose the best spec for the job, and when that job is DPS, the optimal choice is Fury for the time being. This isn't to say that Gladiator isn't viable. It's just not optimal.