collections.js

module.exports = {
  "normal": [
    "reset.css",
    "public_styles.css"
  ],
  
  "admin": [
    "reset.css",
    "bootstrap.css",
    "admin_styles.css"
  ]
}

Gulpfile.js

var gulp = require("gulp");
var collections = require("./collections");

gulp.task("concat-css", function() {
    Object.keys(collections).forEach(function(name) {
      gulp.src(collections[name])
        .pipe(plugins.concatCSS("collections/" + name + ".css"))
        .pipe(gulp.dest("assets/stylesheets"));
    });
});