The Google tag (gtag.js), formerly called the global site tag, is Google's JavaScript tag for website measurement. A tag is a small piece of code in a web page that sends data about each visit to an outside service. One Google tag replaces separate tags for Google Ads, Google Analytics, Campaign Manager 360, Display & Video 360 and Search Ads 360.
A site owner copies a tag ID from a Google product such as Google Ads or Google Analytics and pastes it into a short code snippet. The snippet loads gtag.js from googletagmanager.com and runs a config command with the tag ID; it goes on every page to be measured. The tag then sends data to the linked products: page views, clicks and scrolls to Google Analytics, and conversions and campaign performance to Google Ads. One tag ID can connect to several destinations, and its settings can be changed later in the Google tag screen of those products without new code.
Google's pages state no price for the tag itself; it is set up from Google Ads, Google Analytics and Campaign Manager 360.