This would not be, and I quote, a "jack**s move". Why not, you ask? Because Gravity does not garuntee a hit. Let's see what different sources have to say.
Prevents moves like Fly and Bounce and the Ability Levitate for 5 turns. ~ Pokémon DB
Gravity is intensified for five turns, making moves involving flying unusable and negating Levitate.
Removes Ground-type resistance. Prevents certain moves from being used. Lowers all Pokémon's EVASION two stage. ~ Serebii
The only thing helping Fissure to hit is the -2 Evasion. The calc for the accuracy of Fissure is
UserLevel - TargetLevel + 30
Let's apply that to a Lv. 100 Steelix vs. Lv. 100 Marowak.
100 - 100 + 30 = 30
Wouldja look at that, the accuracy is still 30! Now let's factor in the Evasion. At -2, the chance of hitting increase by 60% (Source), so let's find 60% of 30.
30 x 0.1 = 3 (This was just a stepping stone. 3 is 10% of 30.)
3 x 6 = 18 (Now we have 60% of 30. Let's make one more calc, just to make sure.)
18 ÷ 30 = 0.6
0.6 x 100 = 60 (Voila.)
Okay, so the accuracy of Fissure is 30 normally, and the -2 Evasion adds another 18% accuracy. Next we'll add the two and get our hit/miss ratio.
30 + 18 = 48
Now we know that Fissure has a 48% chance of hitting. Of course, that means that there is also a 52% chance of it missing. So approximately 50/50. It's an improvement on 30/70, but not much of one. So your Fissure strategy is shaky at best. If you want the exact hit-to-miss ratio, it is 12:13.
There, now I'm done presenting my first argument for how your strategy is unviable. Next I will present my second (thankfully shorter) reason.
Fissure has a PP of 5, 8 if maxed. In a Triple Battle there are six opponents, and in Doubles four. Let's apply the hit ratio and miss ratio to Fissure's PP.
8 x 0.5 = 4
8 x 0.02 = 0.16 (pre-calculations)
4 - 0.16 = 3.84 (hits)
4 + 0.16 = 4.16 (misses)
What does that tell us? It tells us that out of 8 uses, Fissure is going to hit 3.84 times and miss 4,16 times. This leaves us with one important question, one that we must solve: What is the probability of Fissure OHKOing all of the opposing Pokémon in a Double/Triple Battle? Well, there is a near-clean 50% chance of hitting all four opposing Pokémon in Doubles. The math is simple as pi: (<- see what I did there?)
8 ÷ 4 = 2
4 ÷ 2 = 0.5 (four is used because 4:2 is the exact same as 8:4)
Woah. 8 PP divided by 4 Pokémon equals 50%. Favourable? Not really. Triples is even worse.
8 ÷ 6 = 1.33333…
4 ÷ 1.3… = 0.3… (four is used because it was used in the previous calc)
Yowzah. 8 PP divided by 6 Pokémon equals a whopping 33%. That doesn't look good. 50% for Doubles and 33% for Triples. And that's just the mathematical probability. In practicality, it could hit 1/8 times or miss 5/8 times. It's really just the luck of the draw, and that's not something you should be relying on in competitive battling if the probability of missing is below 30% (see Focus Miss for details).
TL;DR It is a plausible strategy, but is my no means a practical one.