Google isn't the one asking us for the SSO service URL, Laserfiche is the one asking for it; on the SSO config page. I checked under advanced and it says Enable IDP-initiated login flow = Yes but IdP-initiated Identity Providers do not provide this service URL.

Google expects that the IdP will handle initiating the authentication request (IdP-initiated flow), so Google doesn't need an SSO Service URL to handle login requests from its side.

Instead, Google focuses on the Assertion Consumer Service (ACS) URL, which is the endpoint where the IdP sends the SAML response after a user is authenticated.

Where is that quoted section from? Please provide a link if it's a Google documentation resource.

Google's Set up your own custom SAML app Using SAML-based SSO says plainly that it provides an IDP SSO URL:

1. (5.) On the Google Identity Provider details page, get the setup information needed by the service provider using one of these options:
   1. (a.) Download the IDP metadata.
   2. (b.) Copy the SSO URL and Entity ID and download the Certificate (or SHA-256 fingerprint, if needed).

And I assure you that Google does not only support IDP-initiated SAML SSO flows. They literally provide instructions on testing both IDP and SP-initiated flows on their SAML config page (see below), and I've worked with customers to set up Google SAML SSO with both SP and IDP-initiated flows enabled with Directory Server. It may be that your customer has somehow disabled SP-initiated SSO at an administrative level in Google Workspace, but it is certainly supported.

If you can't find the IDP SSO URL value in the Google UI, try downloading the IDP metadata and pasting its contents into the Laserfiche Cloud "Import Metadata" dialog, Import, and see if the value populates: