In order to publish to a repository in Maven format ,“maven-publish” plugin for gradle can be used.

The plugin should be added to build.gradle file in library module.

apply plugin: 'maven-publish'

You should define the publication and its identity attributes in build.gradle file too. This identity attributes will be shown in the generated pom file and in future for importing this publication you will use them.You also need to define which artifacts you want to publish,for example i just want to publish generated .aar file after building the library.

publishing {
    publications {
        myPulication(MavenPublication) {
            groupId 'com.example.project'
            version '1.0.2'
            artifactId 'myProject'
            artifact("$buildDir/outputs/aar/myProject.aar")
        }
    }
}

You will also need to define your repository url

publishing{
    repositories {
        maven {
            url "http://www.myrepository.com"
        }
    }
}

Here is full library build.gradle file

apply plugin: 'com.android.library'
apply plugin: 'maven-publish'

buildscript {
   ...
}
android {
    ...
}
publishing {
    publications {
        myPulication(MavenPublication) {
            groupId 'com.example.project'
            version '1.0.2'
            artifactId 'myProject'
            artifact("$buildDir/outputs/aar/myProject.aar")
        }
    }
    repositories {
        maven {
            url "http://www.myrepository.com"
        }
    }
}

For publishing you can run gradle console command

gradle publish

or you can run from gradle tasks panel