Stall isn't threatened at all by any version of Garchomp, because Skarmory wals it hardcore. Even YacheChomp, or SD + 3 attacks as I prefer to call it, is not a guaranteed 2HKO on Skarmory, doing between 48.5% - 57.49% per hit. If it's Chomp v. Rain Stall, then obviously Skarm's not going to be threatened by Fire Fang at all (another case for using Aqua Tail imo). I don't know what SD Chomp really threatens, to be honest...every team I've ever made has some way or another of handling it. No reason for the Rough Skin version to not be OU. Sand Veil, of course, is a much different story.
Some calcs that might interest you.
252 +2 Atk Life Orb Garchomp Fire Fang vs 252 HP/252 Def Skarmory: 63.47% - 74.25%
That's an easy 2HKO, unless Skarmory is in rain obviously, but if Garchomp is being run on a sand/sun team....
252 +2 Atk Life Orb Garchomp Fire Fang vs 252 HP/252 Def Skarmory: 94.61% - 110.78% (in sun)
Yea, ouch.
252 +2 Atk Life Orb Garchomp Aqua Tail vs 252 HP/252 Def Skarmory: 64.97% - 76.65% (in rain)
Rain stall skarmory can't handle chomp either... (I've been running Chomp on a rain team and it does wonders with Pseudo-STAB aqua tail)
252 +2 Atk Life Orb Garchomp Outrage vs 252 HP/252 Def Skarmory: 43.11% - 50.9%
That's a possible 2HKO after stealth rocks btw.
So we have the sturdiest physical wall in the game easily falling to chomp, great counter? (skarmory normally runs 232 def anyway, so it's taking the attacks slightly worse than that).
Forretress gets easily 2HKO'd by +2 EQ while failing to do anything significant back.
0 SpAtk Forretress Hidden Power Ice vs 4 HP/0 SpDef Garchomp: 43.58% - 51.4%
92 SpAtk Forretress Hidden Power Ice vs 4 HP/0 SpDef Garchomp: 50.28% - 59.22%
That's a lot of wasted EVs to get the 2HKO >.>
252 +2 Atk Life Orb Garchomp Earthquake vs 252 HP/88 Def Ferrothorn: 81.25% - 95.45%
252 +2 Atk Life Orb Garchomp Fire Fang vs 252 HP/88 Def Ferrothorn: 140.91% - 165.91%
252 +2 Atk Life Orb Garchomp Earthquake vs 252 HP/252 Def Ferrothorn: 71.88% - 84.38%
252 +2 Atk Life Orb Garchomp Fire Fang vs 252 HP/252 Def Ferrothorn: 125% - 147.73%
Ferrothorn isn't at all taking on chomp either, specially defensive sets lose even worse.
252 +2 Atk Life Orb Garchomp Earthquake vs 252 HP/4 Def Ferrothorn: 95.45% - 111.93%
Even if you succeed in getting chomp to get himself locked into outrage....
252 +2 Atk Life Orb Garchomp Outrage vs 252 HP/4 Def Heatran: 62.44% - 73.58%
252 +2 Atk Life Orb Garchomp Outrage vs 252 HP/4 Def Jirachi: 62.87% - 74.01%
252 +2 Atk Life Orb Garchomp Outrage vs 252 HP/88 Def Ferrothorn: 48.58% - 57.1%
(likely 2HKO after rocks.....assumign ferro is completely healthy)
252 +2 Atk Life Orb Garchomp Outrage vs 252 HP/252 Def Ferrothorn: 43.18% - 50.85%
Takes it the best, but has no reliable recovery......
And just for fun
252 +2 Atk Life Orb Garchomp Outrage vs 252 HP/252 Def Tangrowth: 77.48% - 91.58%
252 +2 Atk Life Orb Garchomp Outrage vs 252 HP/252 Def Slowbro: 87.06% - 102.79%
252 +2 Atk Life Orb Garchomp Aqua Tail vs 252 HP/252 Def Hippowdon: 78.1% - 91.9%
252 +2 Atk Life Orb Garchomp Earthquake vs 252 HP/252 Def Hippowdon: 65% - 76.43%
So basically, stall has nothing that can take a hit from chomp, that's just plain to see.