Build an AI Text Rewriter in Google Sheets Using Gemini and Apps Script

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Artificial intelligence is no longer limited to chat interfaces — you can now embed powerful AI tools directly into your favorite Google Workspace apps. In this tutorial, we’ll walk through how to build an AI Text Rewriter inside Google Sheets using Google Apps Script and the Gemini REST API.

GitHub https://github.com/lsvekis/Google-Apps-Script-Gemini-Projects/tree/main/AI%20Text%20Rewriter%20in%20Google%20Sheets

By the end, you’ll have a fully functional project that lets you rewrite sentences, paragraphs, and full articles directly inside a spreadsheet — all powered by Google’s Gemini model.

Build an AI Text Rewriter in Google Sheets Using Gemini and Apps Script (1)Download

🧩 What You’ll Build

Your final project will include:

  • A custom menu called “🧠 AI Tools”
  • A sidebar interface to rewrite text in different tones (e.g., professional, friendly, simplified)
  • A custom function =AI_REWRITE() that works inside any cell
  • Integration with the Gemini REST API using Apps Script’s UrlFetchApp

Here’s a sneak peek of what it looks like:

A spreadsheet where column A contains original sentences and column B auto-generates rewritten versions via Gemini.

🚀 Step 1: Set Up the Project

  1. Open a new Google Sheet.
  2. Go to Extensions → Apps Script.
  3. Delete any existing code.
  4. Create three files:
    • Code.gs — main backend logic.
    • Sidebar.html — user interface.
    • appsscript.json — project manifest (we’ll enable it later).

💻 Step 2: Write the Backend (Code.gs)

Paste the following code in Code.gs:

/**
 * AI Text Rewriter for Google Sheets using Gemini REST API
 * Author: Laurence “Lars” Svekis
 */

const GEMINI_API_KEY = 'YOUR_API_KEY_HERE';
const GEMINI_MODEL = 'gemini-2.5-flash';
const BASE_URL = 'https://generativelanguage.googleapis.com/v1beta';

// Menu setup
function onOpen() {
  SpreadsheetApp.getUi()
    .createMenu('🧠 AI Tools')
    .addItem('Open Rewriter Sidebar', 'showRewriterSidebar')
    .addItem('Insert Sample Data', 'insertSampleData')
    .addToUi();
}

// Sidebar launcher
function showRewriterSidebar() {
  const html = HtmlService.createHtmlOutputFromFile('Sidebar')
    .setTitle('Gemini Rewriter');
  SpreadsheetApp.getUi().showSidebar(html);
}

// Gemini REST call
function callGeminiRewrite(text, style = 'professional tone', temperature = 0.6) {
  const url = `${BASE_URL}/models/${GEMINI_MODEL}:generateContent?key=${GEMINI_API_KEY}`;
  const payload = {
    contents: [
      { role: "user", parts: [{ text: `Rewrite the following text in a ${style}:\n${text}` }] }
    ],
    generationConfig: {
      temperature,
      maxOutputTokens: 512
    }
  };

  const options = {
    method: 'post',
    contentType: 'application/json',
    payload: JSON.stringify(payload),
    muteHttpExceptions: true
  };

  try {
    const res = UrlFetchApp.fetch(url, options);
    const data = JSON.parse(res.getContentText());
    return data?.candidates?.[0]?.content?.parts?.[0]?.text || '⚠️ No rewrite returned.';
  } catch (e) {
    return '❌ Error: ' + e.message;
  }
}

// Custom formula: =AI_REWRITE(A2, "friendly tone")
function AI_REWRITE(text, style) {
  return callGeminiRewrite(text, style || 'neutral tone');
}

// Inserts sample rows
function insertSampleData() {
  const data = [
    ['Original Text', 'Rewritten Text'],
    ['Our company values innovation and collaboration across all departments.', ''],
    ['This policy ensures all employees have equal access to training opportunities.', ''],
    ['Please submit your report by Friday at noon.', '']
  ];
  const sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  sheet.clearContents();
  sheet.getRange(1, 1, data.length, data[0].length).setValues(data);
  SpreadsheetApp.getActive().toast('✅ Sample data inserted');
}

🧠 Step 3: Create the Sidebar (Sidebar.html)

This interface lets users rewrite text manually without formulas.

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <style>
      body { font-family: system-ui; padding: 16px; }
      textarea { width: 100%; height: 100px; }
      button { background: #1a73e8; color: white; border: 0; padding: 8px 12px; border-radius: 6px; cursor: pointer; }
      select, input { width: 100%; margin: 4px 0; padding: 4px; }
      pre { background: #f1f3f4; padding: 8px; border-radius: 6px; white-space: pre-wrap; }
    </style>
  </head>
  <body>
    <h2>🧠 Gemini Rewriter</h2>
    <textarea id="text" placeholder="Enter text to rewrite..."></textarea>
    <label>Choose Tone:</label>
    <select id="tone">
      <option>professional tone</option>
      <option>friendly tone</option>
      <option>simplified explanation</option>
      <option>marketing tone</option>
      <option>academic tone</option>
    </select>
    <label>Temperature (creativity):</label>
    <input type="number" id="temp" min="0" max="1" step="0.1" value="0.6">
    <button onclick="rewrite()">Rewrite</button>
    <h3>Result:</h3>
    <pre id="output"></pre>

    <script>
      function rewrite() {
        const text = document.getElementById('text').value;
        const tone = document.getElementById('tone').value;
        const temp = parseFloat(document.getElementById('temp').value);
        document.getElementById('output').textContent = '⏳ Rewriting...';
        google.script.run
          .withSuccessHandler(r => document.getElementById('output').textContent = r)
          .callGeminiRewrite(text, tone, temp);
      }
    </script>
  </body>
</html>

🧾 Step 4: Add the Manifest (appsscript.json)

{
  "timeZone": "America/Toronto",
  "exceptionLogging": "STACKDRIVER",
  "runtimeVersion": "V8",
  "oauthScopes": [
    "https://www.googleapis.com/auth/spreadsheets",
    "https://www.googleapis.com/auth/script.container.ui",
    "https://www.googleapis.com/auth/script.external_request"
  ]
}

This ensures your script can:

  • Modify Sheets data
  • Show sidebars and dialogs
  • Make external HTTP requests to Gemini

🧰 Step 5: Run and Authorize

  1. In Apps Script, click Run → onOpen().
  2. Approve the authorization scopes.
  3. Reload your spreadsheet.
  4. You’ll see a new menu: 🧠 AI Tools.

From there:

  • Insert Sample Data to test
  • Open Rewriter Sidebar to experiment with custom tones
  • Try =AI_REWRITE(A2, "friendly tone") in any cell

⚙️ How It Works

1. UrlFetchApp.fetch()

This function sends an HTTP POST request to Gemini’s REST endpoint:

POST https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent?key=API_KEY

The payload defines:

  • contents — the user’s input message.
  • generationConfig — creativity (temperature) and token length.

2. Parsing the Response

Gemini returns a JSON object.
The relevant text is usually found at:

data.candidates[0].content.parts[0].text

If Gemini detects unsafe or truncated content, the function returns helpful messages instead of errors.

3. Reusable Functions

You can:

  • Wrap Gemini calls into your own custom formulas (AI_SUMMARY, AI_REWRITE, AI_TRANSLATE).
  • Use them in automation, workflows, or classroom exercises.

🧠 Learning Outcomes

After completing this exercise, you now understand:

  • How to connect Google Sheets to Gemini’s REST API
  • How to send structured prompts using Apps Script
  • How to create UI extensions (menus + sidebars)
  • How to handle and display AI output in Sheets

This pattern forms the foundation for:

  • Summarizers
  • Grammar Correctors
  • Tone Converters
  • AI-Powered Report Generators