1.

How have others handled this? Do you simply keep them in the roster, or do you periodically audit the roster and remove users as needed?

Yes, they stay active unless I know 100% that they left and decide to deactivate their profile. But that’s rare. It’s not an issue for us bc it sounds like we’re not prioritizing the same metrics you are around engagement (instead we look at total active users each day, week, month, quarter, etc … not % of total).

One tip from my experience: I use the Ban User feature here. And I’ve updated the Phrase they see when trying to log-in to say that their credentials are outdated and should reach out. We use SSO too, but there’s a small loophole where some try to use old credentials for community via a training sandbox where their old username and pw still may be active.

2.

Also, in SFDC we can mark a user attribute called “No Longer with Company”. Can we utilize the SFDC x Gainsight integration so that anyone who has this attribute checked who is also a registered community user would be removed from the roster?

The native integration can’t do this for you, but you can use an automation tool like Zapier or Workato. I have a number of automations that push things to Gainsight CC based on a change in Salesforce. My colleagues helped with it a bit, but my understanding is that you’d need two parts

1. Salesforce automation that pushes a change in the Contact record (no longer with company) and updates the associate Community Member object created for the integration. 
2. An automation that triggers when the object is updated and updates the community profile as well. 

Now that I see that typed out, you could probably do it all in one automation, but I think it’s a good best practice to also reflect it on your SFDC Community Member object in case you do any reporting there.