I have written as small springboot app that does basic CRUD(GET,POST,PUT,etc). Is it possible to host it using githubpages or github-pages can be used only for deploying simple html pages?
No, you can not. GitHub pages can only host static content (HTML, CSS, JS, text). This excludes any runtimes, such as the JVM required for Java or NodeJS.
GitHub Pages is a static site hosting service that takes HTML, CSS, and JavaScript files straight from a repository on GitHub, optionally runs the files through a build process, and publishes a website. [emphasis mine]