Before we dive into the spring cloud gateway, you have the chance to get a first glimpse on the difference between imperative and reactive programming style.
To do this we will look into the project reactive-playground.
The following resources might be helpful for first steps into the reactive world:
This playground contains a starting point to play around with the reactive stuff.
Just execute tests in the class BasicReactivePlaygroundTests to learn more about the reactive streams api.
If you want to learn more about the project reactor and reactive streams you may also want to check another GitHub repository for a Reactive Spring Security 5 Workshop.