Seismic Toss? Given Gravity's acc boosting nature, you could put Thunder in that slot. Mine runs Aromatherapy there because I primarily use Bliss as an in-and-out cleric, although it makes for an easy Gravity setup and its alot easier to kill Chompy with Blizzard than Ice Beam.
Course, my team also has Mamoswine, which abuses Gravity just as badly as Garchomp (they have the same base attack) and also forces it out or destroys it. If Garchomp is stuck in Outrage it's finished. Anyway, Garchomp is lethal, but its still quite overhyped. You have to account for it of course, but it's still one pokemon, with a very bad weakness and few good resists.
My Gravity team also had Mamoswine (and Garchomp for that matter), and sure it can force Garchomp out, but that won't stop Garchomp from coming out the next time Gravity is set up. Not to mention Mamoswine will be having troubles switching into Garchomp, so you'll probably have to sacrifice something to bring Mamoswine in the first time. If your typical opponent (which might carry Rhyperior too) can trouble you this much, I just don't see Gravity teams becoming too popular. It's pretty much setting up the field for your opponent to sweep imo =/
Thunder > Seismic Toss gives Electivire and Infernape free switch-ins, not to mention you won't be able to do a thing to Magnezone. And although I'm not a fan of Trick Room ether, unless for some reason your opponent is wierd and decided to make a mono-slow team, Trick Room will be helping you much much more then your opponent. Not to say Gravity sucks though, no, if you can get your Earthquakers in fast enough it can be gg for your opponent! It's just much easier for your non-gravity team to abuse then other field conditions.
Assuming I'm calculating this correctly, Gravity will only make Dynamicpunch 80% or so accurate, not something I'd want to rely on.
This is turning more into a Gravity discussion then a Probopass moveset discussion though, so I'm going to stop before this topic is locked.