Which stats should I add to as an Outlander using Shotguns/Canons?

It depends on how you want to build it. Indeed, in Torchlight 2, the stats are not the usual "obvious" (strength for melee, intelligence for mages, etc.).

From this wiki:

• Strength

Increases all weapon damage
Increases damage done by critical strikes
Note that "all weapon damage" includes all weapons, including ranged weapons and weapons that do elemental damage such as staves and wands.

• Dexterity

Increases critical strike chance
Increases dodge chance (the chance to evade an attack)
Increases fumble recovery (reduces the damage penalty for fumbled attacks)

• Focus

Increases maximum mana
Increases all elemental damage
Increases execute chance (the chance of attacking with both weapons simultaneously when dual wielding similar weapons)

• Vitality

Increases maximum health
Increases armor
Increases block chance (chance to block when using shields)

As an Outlander fighting on range, I assume you will want to maximize your critical damage:

• Strength will increase your damage (raw and critical)
• Dexterity will increase your critical chance

Another aspect to keep in mind is the weapon requirement.

• Shotgonnes usually have a stat requirement on dexterity.
• Cannons usually have a requirement on the strength (and sometimes vitality).

So depending on which one you will favor, you can set your attributes more in one or the other direction.