Dynamic Customer Lists

It would be great to have a way to create simple, dynamic lists of customers. So often our team is creating groups customers that might not have a common field in Gainsight or Salesforce. In these cases it would be great to be able to simply add them to a list - much in the same way that you might add songs to a playlist in many music programs
Have you tried using Customer Tags?  I think this might give you the functionality you are seeking.  More info available at https://support.gainsight.com/hc/en-us/articles/207882547-How-to-Use-Tags.