You can build with the following steps:
$ git --version
git version 2.12.0
$> node -v
v9.3.0
$> git clone https://github.com/ionic-team/ionic-native-google-maps
$> cd ionic-native-google-maps
$> npm install
$> npm run build
...
Making new TMP directory
Preparing core module package.json
Building plugin: google-maps
Done processing plugins!
$> cd (your ionic project)
$> npm install (path to ionic-native-google-maps)/dist/\@ionic-native/core --link
$> npm install (path to ionic-native-google-maps)/dist/\@ionic-native/google-maps --link
$> ionic cordova run android (or ios)