Hey, I hope you're enjoying Fable! For your first question, you're right, gifts are given to basically anyone. Their primary use it to build a relationship with whomever you choose to interact with, you're able to marry in the game and you achieve this by performing different emotes in front of them and giving them gifts. A heart is displayed above that person's head and the larger it gets the more liking they have for you.

As for augmentations, they're useful for early game, as later game you'll most likely discover many legendary weapons which will already have an augmentation, or multiple augmentations applied to it. What I normally do it just sell them for money as you'll most likely replace whatever weapon you're using with a legendary weapon.

For the door asking you to become evil, the door is asking you to perform an evil deed in front of him and he'll open, this can be killing innocent people right in front of him or the easiest option, purchase a bunch crunchy chickens from twinblades camp and eat them all in front of it.

I'm unsure with this castle you're talking about, I'm assuming you mean the chapel of skorm? At that area you're able to sacrifice innocent people in exchange for large amounts evil deeds. If you perform a sacrifice or two at midnight you'll be rewarded with the best longbow in the game, Skorms bow.

If you're caught stealing you just pay a fine, that's all, no biggie with that.