Support has shared that the current version of the Gainsight application does not yet support authentication through YubiKey when used in conjunction with the built-in browser.  This idea is to provide support for this configuration.  

Today, we are not able to login to the mobile app due to this issue.  So unfortunately, we are not able to use the mobile app at all.  This limits our ability to encourage the use of Gainsight outside the office.