We migrated to the “On Premise” version of Outlook after we launched Gainsight, which resulted in the Gainsight Assist app failing to map to timeline in the webmail version of Outlook. 

I’m creating this post at the request of the Gainsight support team. However, If anyone has any effective workarounds, that would be helpful. Currently we are directing our staff to use the installed version of Outlook. In case it’s helpful for anyone encountering a similar issue, here are the findings from Gainsight support:

Based on our review, we found that the on-premises restrictions are preventing us from logging email to a timeline since we expect content from the Outlook exchange server in order to do so
At the moment, the GS assist add-in is only compatible with Office 365. Hybrid exchange organizations (mailboxes stored both on-premises and in Office 365) might have worked with our current design, but Microsoft ended support for that too recently due to a decline in on-premises server adoption.
We must develop a new flow for the on-premise editions only to resolve mail content-related issues