Return of Reckoning

Does Shamans toughness debuff from Touch of Gork tactic stack with other toughness debuffs?
like chosens aura, abilities or debuffs from sets?

buff/debuff skills don`t stack with other skills and potion, stack only with morals

Work only the highest effect.

All buffs and debuffs can be looked at from only two points, which I'll call 'internal' and 'external'. We'll use your question and toughness as the example, but this applies to all stats, armor, resists.

Internal: your gear, your talisman, your tactics, your rr points, morales.
External: abilities (yours or others [friend or foe]), potions, auras.

E.g. you have 500 toughness from your gear and talisman, you cast DSU, giving your an exta say 150 toughness for a total of 650. A knight with max vigilance and Stand Strong! joins the fight and you have a -120 toughness applied, bringing your effective toughness to (500+150-120) 530. You drink your toughness potion mid battle for an +100 bonus as your DSU wears off, so your toughness is now (500+100-120) 480. You recast DSU for (500+150-120) 530 and you only gain the highest bonus, here DSU is greater than your +100 from the potion. One last part could be some slayer hits you with one stack of Pulverizing Strike giving you a -25 to your toughness. The knights -120 aura is still the greatest so you ignore this until the knight is out of range.

The above is true for any of your stats, armor, or resists. Morales are good because they usually have very high values, potions are good because they can't be removed so you don't have 'down time' with them.

havent actually tested it for this but in some cases tactics stack with other things

I did some quick testing on my shaman before posting, so there might be some oddity out there that doesn't follow this, but this is a solid guide.