How can I gzip my index.main.js for flutter web?

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2

You don’t have to do this. Netlify automatically serves HTML, CSS, JS among other files with Brotli compression which is better than GZip in compression ratios. However, the support depends on browsers. Most modern browsers support Brotli, but if they don’t Netlify will serve with GZip. If even GZip isn’t supported by the browser, Netlify will fall back to no compression.