Unless there are some hidden API operations (I don’t really see anything for groups in the API documentation ), you might be out of luck. 

I’d imagine though, that there are hidden API operations that Gainsight might be able to provide. 

If there are, you could export group lists to CSV and then automate the running of the API operation to remove each member one by one. (I can tell you how to do it in Zapier if you get good news on the API operations and have that tool in your portolio)

This is a massive feature gap for really basic group management. I’ll consult with our CSM. Thankfully this isn’t pressing but I sure as heck am not about to individually click and confirm over a thousand users to remove them. 

Worst-case scenario I’ll just move the content into other appropriate topics and delete the group entirely. 

If you’re just removing everyone, you might be able to get support to do it for you. I’d imagine there exists internal functionality that they don’t have public yet. 

Great call, thanks @DannyPancratz!