Unable to resolve dependency for ':app@debug/compileClasspath': Could not resolve com.squareup.retrofit2:converter-gson:2.1.0.

Could not resolve com.squareup.retrofit2:converter-gson:2.1.0.
Required by:
    project :app
 > No cached version of com.squareup.retrofit2:converter-gson:2.1.0 available for offline mode.

从 远程仓库 下载的项目导入本地后出现这个问题, 看到 console 中有提示与 offline 模式有关的提示,反应过来有可能是开启了这个离线模式。于是通过点击 setting → Build, Execution, Deployment → Gradle 找到 Offline Work 模式把打勾去掉即可。