The global site tag (gtag.js) is a JavaScript library that allows you to send event data to Google Analytics. The reports in your property start to show data when you add the global site tag to your web pages. Each page should have only one global site tag. To configure more than one property, add an additional config command.
If you set up Analytics on your site after August 2017, your site is probably tagged with gtag.js. If you set up Analytics on your site before August 2017, your site is probably tagged with analytics.js.