What is the syntax for writing comments in build.gradle file?

Easy:

// Single line comment

/*
 Multi
 line
 comment
*/

Use the // or /* */

For example:

        // Top-level build file where you can add configuration options common to all sub-projects/modules.

    buildscript {
        repositories {
            jcenter()
        }
        dependencies {
            classpath 'com.android.tools.build:gradle:1.2.3'

            // NOTE: Do not place your application dependencies here; they belong
            // in the individual module build.gradle files
        }
    }

/**
 * Returns the credential username used by Namirial Maven repository
 * Set this value in your ~/.gradle/gradle.properties with CREDENTIALS_USERNAME key
 * @return
 */
def getCredentialsMavenUsername() {
    return hasProperty('CREDENTIALS_USERNAME') ? CREDENTIALS_USERNAME : ""
}