@ Spiritomb
against max defense Spiritomb...
Adamant LO +2 Stone Edge vs 252/252+ Spiritomb
766 Atk vs 346 Def & 304 HP (100 Base Power): 206 - 243 (67.76% - 79.93%)
Adamant LO +2 Shadow Sneak vs 252/252+ Spiritomb
766 Atk vs 346 Def & 304 HP (40 Base Power): 83 - 98 (27.30% - 32.24%)
which is an essentially guaranteed 2HKO after SR + Leftovers (minimum 95.06%)
meanwhile, Spiritomb can't do much back to Gallade (it only gets one Shadow Sneak since Gallade will Shadow Sneak itself the second turn)
0 Atk Spiritomb Shadow Sneak vs 0/0 Gallade
220 Atk vs 166 Def & 277 HP (40 Base Power): 116 - 138 (41.88% - 49.82%)
since Stone Edge + Shadow Sneak nearly always KOes 252/252+ Spiritomb, Stone Edge + Burnt Stone Edge will always KO (100 + 40 vs 100 + 50).
if you decide to EV your Spiritomb offensively...
252+ Atk Spiritomb Shadow Sneak vs 0/0 Gallade
331 Atk vs 166 Def & 277 HP (40 Base Power): 170 - 204 (61.37% - 73.65%)
Adamant LO +2 Stone Edge vs 252/0 Spiritomb
766 Atk vs 252 Def & 304 HP (100 Base Power): 283 - 334 (93.09% - 109.87%)
whichever way you EV Spiritomb, it's dying before Gallade dies. note that 252 HP / 252+ Def Spiritomb has significantly less staying power against many of the other things it's supposed to check, like Rotom, Mismagius, and Alakazam, which means most Spiritombs should be running SpD (which makes it take more damage from Stone Edge relative to the first calculation while still not doing any more damage to Gallade). conclusion: Spiritomb can sacrifice itself to stop Gallade from sweeping an entire team, but you'll probably lose another Pokemon at least if you have no other priority / faster Pokemon.
@ Drapion
Adamant LO +2 Close Combat vs 252/252+ Drapion
766 Atk vs 350 Def & 344 HP (120 Base Power): 366 - 432 (106.40% - 125.58%)
(similarly this isn't a very good spread for stall Drapion because it neglects its good special resistances)
if you decide to EV Drapion offensively then it is an okay check
Adamant LO 252 Atk Crunch vs 0/0 Gallade
306 Atk vs 166 Def & 277 HP (80 Base Power): 205 - 243 (74.01% - 87.73%)
but you still can't switch Drapion into Gallade
383 Atk vs 256 Def & 281 HP (120 Base Power): 250 - 295 (88.97% - 104.98%)
against max defense Spiritomb...
Adamant LO +2 Stone Edge vs 252/252+ Spiritomb
766 Atk vs 346 Def & 304 HP (100 Base Power): 206 - 243 (67.76% - 79.93%)
Adamant LO +2 Shadow Sneak vs 252/252+ Spiritomb
766 Atk vs 346 Def & 304 HP (40 Base Power): 83 - 98 (27.30% - 32.24%)
which is an essentially guaranteed 2HKO after SR + Leftovers (minimum 95.06%)
meanwhile, Spiritomb can't do much back to Gallade (it only gets one Shadow Sneak since Gallade will Shadow Sneak itself the second turn)
0 Atk Spiritomb Shadow Sneak vs 0/0 Gallade
220 Atk vs 166 Def & 277 HP (40 Base Power): 116 - 138 (41.88% - 49.82%)
since Stone Edge + Shadow Sneak nearly always KOes 252/252+ Spiritomb, Stone Edge + Burnt Stone Edge will always KO (100 + 40 vs 100 + 50).
if you decide to EV your Spiritomb offensively...
252+ Atk Spiritomb Shadow Sneak vs 0/0 Gallade
331 Atk vs 166 Def & 277 HP (40 Base Power): 170 - 204 (61.37% - 73.65%)
Adamant LO +2 Stone Edge vs 252/0 Spiritomb
766 Atk vs 252 Def & 304 HP (100 Base Power): 283 - 334 (93.09% - 109.87%)
whichever way you EV Spiritomb, it's dying before Gallade dies. note that 252 HP / 252+ Def Spiritomb has significantly less staying power against many of the other things it's supposed to check, like Rotom, Mismagius, and Alakazam, which means most Spiritombs should be running SpD (which makes it take more damage from Stone Edge relative to the first calculation while still not doing any more damage to Gallade). conclusion: Spiritomb can sacrifice itself to stop Gallade from sweeping an entire team, but you'll probably lose another Pokemon at least if you have no other priority / faster Pokemon.
@ Drapion
Adamant LO +2 Close Combat vs 252/252+ Drapion
766 Atk vs 350 Def & 344 HP (120 Base Power): 366 - 432 (106.40% - 125.58%)
(similarly this isn't a very good spread for stall Drapion because it neglects its good special resistances)
if you decide to EV Drapion offensively then it is an okay check
Adamant LO 252 Atk Crunch vs 0/0 Gallade
306 Atk vs 166 Def & 277 HP (80 Base Power): 205 - 243 (74.01% - 87.73%)
but you still can't switch Drapion into Gallade
383 Atk vs 256 Def & 281 HP (120 Base Power): 250 - 295 (88.97% - 104.98%)