Google just completely transformed the Antigravity AI coding agent with Gemini 3.1, and in this tutorial I show you how to use it to build a real, production-style full‑stack SaaS app—not some random MVP that breaks the second you touch it. We build an appointment booking app (think Calendly) with a modern React frontend, light/dark mode, a client dashboard for managing bookings, and an admin panel where I can view upcoming sessions, edit/block my schedule, track payments/clients, and even get an AI summary at the top. I walk you through the exact workflow I use: generate a fully functional demo shell first (everything clickable, forms working, fake payments), test locally with Node/npm, then push to GitHub and deploy to Vercel with a custom domain so you can test real flows early. After that, we add a reliable backend with Firebase: Authentication (email/password, Google, and anonymous users), Firestore database structure, and strict Security Rules to handle user roles (I keep it simple with a role field). Finally, I explain why Cloud Functions are crucial—stop exposing API keys in the browser—so you can safely run AI features, handle PayPal payments, and send emails (like confirmations) from the backend.