You still need to add Google Analytics to GTM.
GTM makes it easy to add Javascript to the HEAD section of your website. If you weren’t using GTM, you’d need to manually edit your website’s HTML to add the new Javascript. However, with GTM, you just update your GTM settings and your website automatically gets the new Javascript.
You can think of GTM as a container which will run whatever Javascript you want to run on your website.
Since you want to use Google Analytics, you need to add the Google Analytics Javascript to GTM. There are many tutorials explaining how to do this.