My husband is out of storage but he is a member of Family Sharing and we have storage available, what to do?

Google services like Gmail have nothing to do with iCloud+. They are entirely separate companies and services.

Gmail uses Google's own storage not iCloud. If he's out of storage on Google / Gmail, then he needs to either delete content from Google, or buy more storage from Google.

click here ➜ https://support.google.com/mail/answer/9312312?hl=en

Google supplies 15gb of its own cloud storage for free (unrelated to Apple’s iCloud), so your husband will need to trim the content of his Google account or buy more space from Google. His Google Drive may have content he can store elsewhere. He can also consider deleting emails with large attachments (both in the inboxes and Sent Messages) remembering to empty Google’s trash folders too. Hope that helps

If it is Google complaining about storage then it is his storage on Google's service, not Apple's. Still, you get 15 GB of storage with Gmail so he must have an astronomic number of emails if he has managed to take up the full 15 GB.

Anyway, he may have to start subscribing to a higher Google service. Again, nothing to do with Apple unless the message is actually from Apple. If it is then, read this support article for instructions on how to upgrade to iCloud+ --> Upgrade to iCloud+ - Apple Support

- The article includes instructions for various hardware and managing your plan.