总结一下需要掌握的点：
Groovy基础语法
Gradle生命周期
Task
Plugin
邓凡平 深入理解Android:Gradle详解
http://www.infoq.com/cn/articles/android-in-depth-gradle#anch130838
任玉刚
Gradle从入门到实战－Groovy基础
https://blog.csdn.net/singwhatiwanna/article/details/76084580
全面理解Gradle－定义Task
https://blog.csdn.net/singwhatiwanna/article/details/78898113
全面理解Gradle－执行时序
https://blog.csdn.net/singwhatiwanna/article/details/78797506
最关键的学习方式就是看官方文档说明：
https://docs.gradle.org/current/dsl/
http://docs.groovy-lang.org/latest/html/groovy-jdk/
https://docs.gradle.org/current/javadoc/org/gradle/api/Project.html
http://google.github.io/android-gradle-dsl/current/index.html
http://blog.bugtags.com/2016/03/28/embrace-android-studio-gradle-plugin/
Plugin & Sample
https://docs.gradle.org/current/userguide/custom_plugins.html
https://github.com/gradle/gradle/tree/master/subprojects/docs/src/samples