Thanks for writing in!
Most of the time if there is a need to insert a code in the header or footer, we recommend to install "Insert Headers and Footers" plugin (https://wordpress.org/plugins/insert-headers-and-footers/). This plugin will allow you to easily manage any script you may need to add for the header or the footer. This plugin may not apply if the script or code added is for specific pages.
To resolve your issue, I would recommend that you install a child theme. What you are trying to accomplish requires a template customization, we would highly to suggest that you use a child theme. This allows you to make code changes that won’t be overwritten when an X update is released.
Once the child theme is set up, please add the following code in your child theme’s functions.php file
if ( is_page(123) ) : ?>
<p>Add your Google Site Tag code here</p>
Please noticed that I have a condition
is_page(123) where in
123 is the page ID where you want the code to insert. To know more how you can get the page ID, please check this out: https://theme.co/apex/forum/t/setup-how-to-locate-post-ids/59
Please let us know how it goes.