What are some alternatives?
When comparing Gem.js and Polymer, you can also consider the following products
Vue.js - Reactive Components for Modern Web Interfaces
AngularJS - AngularJS lets you extend HTML vocabulary for your application. The resulting environment is extraordinarily expressive, readable, and quick to develop.
Svelte - Cybernetically enhanced web apps
Preact.js - Preact is a fast 3kB alternative to React with the same modern API. Components & Virtual DOM.