I don’t believe this works “by default”, because with Gainsight Assist, you have to authenticate to Gainsight using some Gainsight user. 

I would be curious if anyone has hacked this one though, and found some creative work-around or configuration to support this use case.

I think it should be feasible ,if we create a generic user and authenticate the shared mailbox using it . I have not tried but logically looks doable.

Unfortunately, it looks like the Gainsight Assist Plugin does not show up in the shared inbox at all. 

It has worked for us but Gainsight Assist would have to use a user account in GS to authenticate. I’m not willing to give out a shared login, so our CSMs still have to log emails as them to Timeline.