Google Tag vs. Google Tag Manager vs. gtag.js
Three different names for ways to install the same tracking, and mixing them up is a common reason conversion tracking never gets set up.
What it means
Conversion tracking can be installed a few different ways: gtag.js is code placed directly on your pages; Google Tag Manager (GTM) is a separate container tool that manages and fires that code (and others) without you editing your site's code each time; and "the Google tag" is Google's current umbrella term for the tracking snippet itself, however it gets installed. They're not competing systems — GTM is a way of managing the same underlying tag, not a replacement for it.
Why it matters
This is one of the most common places self-serve conversion tracking stalls out: someone installs gtag.js directly, later adds Google Tag Manager thinking it's a separate requirement, and ends up with either duplicate tags firing or confusion about which one is actually live.
ask datanephew
Do I need Google Tag Manager, or is the tag on my site enough?
Ask datanephew to check whether your site's tag is actually firing and recording activity. If it is, you don't need to add anything else — Tag Manager is a convenience for managing tags, not a separate tracking requirement.