How to refer code from one independent gradle project to another gradle project?

Project 
  |- api-gateway  (package)
  |- base-service (package)
  |- service-discovery (package)
  |- user-service (package)

This is my project structure.

There is no root project. Every project is independent. base-service has UserModel class. I want to use that UserModel class inside the user-service. I want to avoid duplicate code so I kept all model classes in base-service. When we build user-service it should refer to the base-service code and include that also in the JAR. How to achieve it?

One way I got from the internet is by mentioning like this:

includeBuild(../base-service)

in settings.gradle of user-service and product-service. But still, I am not able to use any class from base-service.

In the root directory of your build, you should add a settings.gradle file, where you declare all your (sub)projects in your build. This would look like this with your structure:

include 'api-gateway'
include 'base-service'
include 'service-discovery'
include 'user-service'

You need now in every subproject a 'build.gradle' file (I assume you have already one), where you can declare the dependencies inside your build, e.g. the build.gradle file in your user-service directory contains

...

dependencies {
  implementation project(':base-service')

  // other dependecies
  
}

...

This will make the classes from base-service available in user-service.