Do I need GCP? How do I know if I'm using it? Is it safe to ignore the Upgrade-your-account warning?

[cnd]

I'm new to all this - so please be gentle 🙂

I wrote and published a gmail add-on which already appears in the "Google Workspace Marketplace" store, is working great, and has active users for more than a week now.  I don't know whether or not that is anything to do with GCP - is it?

I do not think I have ever opened any GCP trial - at the top of most of my workspace admin screens, there is a message which always says "Free trial status: $576.75 credit and 30 days remaining - with a full account, you'll get unlimited access to all of Google Cloud Platform." and a blue "activate" button which I have never clicked.  As far as I remember, it always says the same - 30 days - and it's said that for more than 30 days now. 

I just got the following email from Google Cloud Platform <CloudPlatform-noreply@google.com>:-

30 days left.
Your free trial will expire in 30 days. Keep building on the infrastructure trusted by brands like Snap Inc., Coca-Cola, and others. Upgrade your account now to enable automatic billing and prevent loss of service when your trial expires.  _Upgrade Now_

Thus my main question: how do I know if I am using GCP?

Do I need some kind of subscription to use "Google Workspace Marketplace" ?  (since I'm already using it, I'm wondering it's working because I've maybe been given some kind of workspace trial already?)

I've spent the last hour trying to find somewhere in my console etc which might show billing records, but no luck.  Assuming I do need to pay for something, and assuming I might already have a trial (both of which I don't know if is true or not) - is there a way to see what is being billed to my free trial?  If there is, could someone maybe provide some clues (preferably easy enough for an idiot, me, to follow!) about how I find where that might live?  The closest I've managed to get is on the page "pay.google.com" where it says I've got this:-

Subscriptions and services
Google Cloud xxxxxx-xxxxxx-xxxxxx
Your balance A$0.00 No balance due
Automatic payments
You won’t be charged until there’s a balance due. Your entire A$100.00 payment threshold is available.

Transactions
You don't have any transactions yet

I have no idea if that's relevant, but all the GCP and workspace related help pages I've found have no links going to billing, and their search system says "0 results" when I look for billing...

*Heaps* of thanks in-advance for anyone who might be able to shed light on this!

Also - if anyone in google is reading this - be aware that you have dozens of places where you say that billing support is available for free-trial users, but all of that is not true: it's an endless loop of links that ultimately lead to a place which says no support of any kind is available to free-trials.  Please make up your mind who you're going to provide billing support to, and please fix all your pages to either tell us there is no support, or, make them go somewhere that actually offers support (and no - I don't mean a web page which gives us links to a half-dozen irrelevant pre-written answers with no "other" option - I mean actual real human support).