You can create a simple custom module with inside Vendor/Module/view/frontend/layout/default.xml

....
<referenceContainer name="after.body.start">
    <block class="Magento\Framework\View\Element\Template" name="google.tag.manager" before="-" template="Vendor_Module::html/tag-manager.phtml" />
</referenceContainer>
...

where tag-manager.phtml is exactly your tag manager snippet.

Now you have that google tag manager snippet is loaded as first block after body open in all pages.

You can create a simple custom module with inside Vendor/Module/view/frontend/layout/default.xml

....
<referenceContainer name="after.body.start">
    <block class="Magento\Framework\View\Element\Template" name="google.tag.manager" before="-" template="Vendor_Module::html/tag-manager.phtml" />
</referenceContainer>
...

where tag-manager.phtml is exactly your google tag manager snippet.

So you'll have that the tag manager snippet is added in all pages as first block after body tag open.

Just create a custom module with view/frontend/layout/default.xml with

....
<referenceContainer name="after.body.start">
    <block class="Magento\Framework\View\Element\Template" name="google.tag.manager" before="-" template="Custom_Template::html/tag-manager.phtml" />
</referenceContainer>
...

where view/frontend/templates/html/tag-manager.phtml is your tag manager snippet.

Now you have that google tag manager snippet is the first block after body open.

You can create a simple custom module with inside Vendor/Module/view/frontend/layout/default.xml

....
<referenceContainer name="after.body.start">
    <block class="Magento\Framework\View\Element\Template" name="google.tag.manager" before="-" template="Vendor_Module::html/tag-manager.phtml" />
</referenceContainer>
...

where tag-manager.phtml is exactly your tag manager snippet.

Now you have that google tag manager snippet is loaded as first block after body open in all pages.