A credit card is issued by a company that is lending you the money, and you pay them back. A gift card is purchased by another person for you, and the money has already been paid. When you use a gift card, you are using the money that has already been paid, and you don't pay anyone back. Usually, a credit card can be used in many places, while a gift card is specific to one store or company. However, some credit card companies now also sell gift cards (for example, Visa gift cards). These can be used to purchase things online or in person just like a credit card - the difference is, the money has already been paid when the card was purchased, so you don't have to pay anyone back later.

When a rate or price is "hiked," it means the price is increased, usually by a lot very quickly.

Yes, you can use flagrant for other nouns, such as "a flagrant fault" or "a flagrant insult" or "a flagrant oversight."

In my experience, all of them are commonly used. Fruit salad is probably the most common. Fruit cup implies that it comes in a cup (and contains fruit salad!)

Your nephew and niece have grown up a lot.

Your interpretation is possible. However, it is common for someone to say this if they see themselves as a nice person but also acknowledge their failings. "I try" in this case is a way of the speaker saying something good about himself while still showing humility.

In the US, we rarely use the word "toilet" for this. Usually, we would say, "Excuse me, can I go to the ladies' room" (or the mens' room) or "Can I go to the restroom?" If it is a classroom of adult students, they usually just say "May I be excused?" or "Please excuse me" or "I'll be right back." Americans usually avoid the word "toilet" and sometimes "bathroom" when they are trying to be polite.