Add head and body function for google tag manager in child theme

If your theme is any good it will have the wp_head hook in header.php. In your child theme’s functions.php you can add an action to include anything you want. Like this:

add_action ('wp_head','wpse359909_add_gtm');
function wpse359909_add_gtm () {
  echo 'whatever code you want to add';
  }