People see faces in a lot of things because we are predisposed to see faces. It's part of being human. Because we are social creatures, seeing reactions on faces is very important to us. Sometimes it can mean the difference between life and death.
So we see faces on Mars, in clouds, and on toasts that have come out of the toaster. Sometimes on pancakes, but rarely, for some reason, in gravy. It's not mystical; it's just the way the brain works.
There is an experiment where people are shown an image that is basically noise. But it's noise that vaguely looks like a dog. Most people don't see the dog. Then they are shown a picture of a dog. Then they are shown the first image again. They see the dog.
There are other experiments where there are a bunch of cartoons depicting "neutral" human interactions. One cartoon, for example, might depict three people standing around a table with no real expressions of emotion on their faces. People are shown images of violence and then shown the cartoons. They are more likely to interpret scenarios where there is aggression when asked to describe what is going on in the cartoons.
It works with any kind of "pre-loading." You can train yourself to expect just about anything, and if you do, you'll find it. It's not magic, but it can be magical.
Regarding gratitude, same thing. You'd be better off, I think, practicing gratitude meditations or just simply asking yourself a lot, "What do I have to be grateful for?" than trying to find and stimulate the "gratitude brain wave."
You'll generate the mental activity of gratitude by simply thinking about things for which you're grateful. The brain is complex, and I bet the mental activity associated with the state of gratitude is complex as well and isn't as simple as being a brain wave, like a gamma wave or other kind of overgeneralization like that. It's probably a bunch of different kinds of electrical activity in different parts of the brain, some of them excitatory and some of them inhibitory.
Remember, dogs may bark when it rains, but that doesn't mean that dogs barking CAUSE the rain.
