Once all services are up (only postgresql is required, but using docker is suggested to consume API with swagger) you can use Gemini as a normal Spring Boot project or application. This means that you have two way.
Run Gemini as a standalone application and use its zero-code REST API development.
Or add Gemini as a classic dependency of your project (with Spring already included) and use it as zero-code platform but with the possibility to interact with the framework: changing APIs life-cycle by using Gemini Events and Callbacks. You can also use Spring as you always do, developing completely custom controllers and code.