fix(gtm): expose gtag globally

[DanLDU]

🔗 Linked issue

Global gtag function not exposed when using GTM via useScriptGoogleTagManager (#523)

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Expose globally the gtag function on the Google Tag Manager Script, useful for consent updates.

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[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/nuxt/scripts/@nuxt/scripts@524

commit: 5ca73a3

[harlan-zw]

@huang-julien I guess we had this at some point and then removed it.

I'm fine with adding it back even if it does end up conflicting with different GTM scripts; what do you think?

[huang-julien]

the reason we removed it was if there's multiple google tag managers. one gtag would overwrite another

[harlan-zw]

This PR seems to go against the documented usage of scripts, which is to rely on the .proxy accessor or using the onLoaded() hook.

Given that gtag is essentially a queue function, though, and doesn't fail if adblockers block GTM then I think we should just merge it as there may be some dependency on it being available on the window accessor in other third-party code.