Let's say I have a creature with Flash (You may play this card any time you could cast an instant) in the graveyard, and I cast something like Goblin Dark-Dwellers, or Torrential Gearhulk, which allow me to cast an instant from the graveyard without paying costs, is the creature with Flash a legal target?
A creature with Flash is not a legal target for an effect that targets an Instant
Phrases like "any time you could cast a Sorcery" or "any time you could cast an Instant" are just shortcuts for timing rules, meaning when you're allowed to cast a card or activate an ability.
304.5. If text states that a player may do something “any time he or she could cast an instant,” it means only that the player must have priority. (...)
Both the cards you mentioned have you target an Instant card in your graveyard, meaning the targetted card needs to be an Instant itself.
It's possible that cards get types that aren't originally printed on the them (most commonly for animated Lands or Artifacts), but that's not what Flash does.
In general, no Magic card or effect grants the Instant or Sorcery types.
No, it is not. "Instant" is a card type, which appears in the type line between the art and the text box. When cards like Goblin Dark-Dwellers or Torrential Gearhulk refer to "Instant cards", they are specifically referring to cards with "Instant" printed in their type line.
The ability Flash changes when you are allowed to play a card, but it does not change what type that card is.