Humans are adaptation executors, not fitness maximizers, so this explanation is useful for evolutionary psychology, but it's not what goes on in people's minds.

Gratitude deals with friendship, not contracts. They are more vaguely defined. With a contract, you both agree beforehand on the exact way each of you will help the other. With friendship, you just generally help each other when one of you needs help and the other is in a position to offer it. You may have earned the help with earlier friendship. Or maybe someone helped you to start a friendship, on the basis that you are likely to pay for it later.

I'm not familiar with gratitude journaling. I guess it would make you feel like you have lots of friends.