Read gatsby's docs. https://www.gatsbyjs.org/docs/debugging-html-builds/     You need to check if window is defined, or move it into componentDidMount or useEffect hook.   import { gsap } from "gsap" import { ScrollTrigger } from "gsap/ScrollTrigger"; import { CSSRulePlugin } from "gsap/CSSRulePlugin"; if (typeof window !== "undefined") { gsap.registerPlugin(ScrollTrigger, CSSRulePlugin); }  
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