Can you gain health besides drinking a health potion?
In World of Warcraft, you can sit and it allows the character to gain health faster. Is there anything like that in Torchlight 2?
There are no passive, out-of-combat health regeneration (w/o skills) like in WoW.
Instead there are many other ways to generate/recover health.
You can gain health from:
Armor and weapons with Life Steal (X Health Stolen per Hit) or Health Regen (+X Health Per Second) item mod. There is also the rare item modifier - Fully Heal Self.
Use an Active skills such as the Engineer's Healing Bot, the Embermage's Death's Bounty, the Outlander's Stone Pact, Berserker's Shadow Burst. There are also passive skills that can heal your character, like the Engineer's Charge Reconstitution and the Berserker's Blood Hunger.
Equip and use spell scrolls, such as Heal Self, Heal All, or Draining Touch. Pets can learn spells, too! The have 4 slots just like you character (I personally prefer to give it summon spells). It frees up some of hot bar slots and mana. (Thanks Dycker)
Pet items can also have the modifier "X Health steal to master on Hit"