library ‘gfortran‘ not found

先安装Apple's Command Line Tools for Xcode

$ sudo xcode-select --install

再下载GNU Fortran binary

Tools - R for Mac OS X - developer's page - GNU Fortan for Xcode

下载完之后双击安装文件安装即可

--------------------------

In order to compile R for macOS, you will need the following tools:

• Xcode developer tools from Apple
  Xcode can be obtained from Apple AppStore and the Xcode developer page. Older versions are available in the "more" section of the Developer pages (Apple developer account necessary). On modern macOS versions you can simply use <