I fully agree with you. After looking for a counter, I fully believe there is no counter under these circumstances.At this point, I'm pretty sure a mon has to either 1) have intimidate or 2) resist Earthquake to even have a chance at countering Landorus-I. Under Gravity, and with Sand Force boosting damage, EQ bulldozes through even the sturdiest walls like Forretress and Ferrothorn.
Further, since the only types that resist EQ in gravity are Bug and Grass, they are covered by SE and HP Ice. To me, the only way you're reliably avoiding 2HKOs is with VERY physically bulky Grass types that still have enough SpDef to avoid the 2HKO from HP Ice (since Bug types won't work because the only one resisting EQ and neutral to SE is lol Heracross).
The only other way to counter is with pokes that are faster than Landorus, can OHKO reliably, and can avoid the OHKO from any of Landy's moves after rocks and weather. This is still an incredibly small list, as it's difficult to find a pokemon with the combination of high speed and defenses that still has enough firepower to OHKO Landorus. Outside of gimmicks like scarfed defensive Suicune (lol) you're not going to find anything that has high enough stats in all of the areas it needs to succeed.
Then again, there's always Whimsicott...
@pwnemon Here are the calcs including Gravity:
Claydol @ Leftovers
252 HP / 252 Def / 4 SpD
Lando's most surefire counter, also spins! What a handy guy. HP ice is a 3hko no matter what, so you can even take your time and spin a little before you dispose of lando with a quick ice beam. Definitely landorus's best counter, surprised he has not been yet mentioned.
Earthquake: 141-166 (43.51 - 51.23%) -- 6.64% chance to 2HKO after Stealth Rock
That's the hardest hitting move Landorus has against Claydol and there is still a chance Claydol can't do anything in return. It is a very small chance, and I do agree he is probably the best "counter" to Gavity Lando, but a hard counter at that.