Inject Google Tag Manager (GTM) script into HTML with Gulp.
$ npm install gulp-gtm --save-dev
var gulp = require('gulp')
var gtm = require('gulp-gtm')
// Usage:
gulp.task('gtm', function(){
gulp.src('./index.html')
.pipe(gtm({containerId: 'GTM-1234'}))
.pipe(gulp.dest('./'));
});