Run code after gulp task done with all files

You could just make a task which depends on html-minify:

gulp.task('other-task', ['html-minify'], function() {
  //stuff
});

You could also listen for the stream end event inside the html-minify task:

gulp.task('html-minify', function(done) {
  var files = [
    relativePaths.webPath + '/*.html',
    relativePaths.webPath + '/components/**/*.html',
    relativePaths.webPath + "https://stackoverflow.com/" + relativePaths.appPath + '/components/**/*.html'
  ];

  var changedFiles = buildMetaData.getChangedFiles(files);

  //TODO: needs to execute only after successful run of the task
  buildMetaData.addBuildMetaDataFiles(changedFiles);
  buildMetaData.writeFile();
  var stream = gulp.src(changedFiles, {
      base: relativePaths.webPath
    })
    .pipe(filelog())
    .pipe(minifyHtml({
      empty: true,
      quotes: true,
      conditionals: true,
      comments: true
    }))
    .pipe(gulp.dest(relativePaths.webPath + "https://stackoverflow.com/" + relativePaths.appPath +  "https://stackoverflow.com/" + relativePaths.buildPath));

  stream.on('end', function() {
    //run some code here
    done();
  });
  stream.on('error', function(err) {
    done(err);
  });
});