The guardians give 2 kinds of gifts.
The first gift is a daily one. You can get it once a day. To get your daily gift, you need to press the cards icon in the top left corner of the guardians window. In the next window, there will be 8 cards: turn 1 of them over to get a random gift.
The second gift is given for assembling guardian collections. To get this gift, you must collect all the guardians of one collection.
The guardians' gifts will be useful for progressing through the game. For example, as a daily gift, you can get energy, gold, or even a ruby!
Some guardians can be obtained by taking part in special mini-events. The guardian you receive will remain yours even after the event is over. But if you don't get a guardian, there are no other ways to do it.