Yup, Gildarts is REALLY Cana's dad. It says that something(I don't remember what) happened to her mom when she was little so she went to join the guild her mother told her her dad was in: Fairy Tail. But when she got there and saw Gildarts she couldn't bring herself to say "Hey! I'm your kid!" because she thought he was such an amazing person that he couldn't bother with such a weak and small girl as herself. So she made it her goal to become an S-class mage so when she did she could finally tell her father that she is his daughter. For a long time it's supposed that he has no idea that she is his daughter but *SPOILERS* in the most recent chapter he's seen coming to save her and the other Fairy Tail members even though he supposedly left the island a while ago and they had no contact with him or the rest of the guild. So, I think that he knows she's his kid and came to save her.
Yes he is it was said right well kind of right after they defeated Master Hades, and when the got back to the guild from the 7 year time skip Gildarts keeps bragging about being Cana's dad, which annoyed Cana.
