The foundation of every GA4 measurement.
We verify that the Google tag script (gtag.js) is present on the page, placed high enough in the <head> to fire early, loaded asynchronously so it doesn't block your site, and that it actually executes a network request to Google's servers when the page loads.
Gtag.js is the file that does all the heavy lifting for GA4. If it isn't loaded or isn't loaded correctly, none of your other tracking matters — no events, no conversions, no audiences, no reports. Even subtle problems (loading too late, loading twice, loading in the body instead of the head) silently corrupt your data.
A Shopify store launches a new theme. The dev forgets to add gtag.js to the product template. For three weeks, every product page view is missing from GA4 — the brand thinks their best sellers have zero traffic and pulls ad spend from them.
A B2B SaaS company loads gtag.js inside their cookie banner. Visitors who don't immediately accept consent never get tracked at all, even after they accept — so the conversion path from anonymous visitor to free trial is invisible.
A news site loads gtag.js at the bottom of <body>. Readers who skim the headline and bounce within 2 seconds aren't counted — making their actual top-of-funnel reach look 30% smaller than reality.