formula in activity field for gainsight user id

Hi Christine, As per current implementation we don't have option to create formula fields. 
Changing this to an idea post and redirecting this to product team.

Hi Christine, I don't think I have enough details to be very specific in my recommendations, but broadly speaking, yes, you can use Gainsight Bionic Rules to write any data (from Gainsight or from SFDC) to an SFDC object (assuming permissions are properly configured). List of actions you can take from a Gainsight Bionic rule: https://support.gainsight.com/Rules_Engine/Admin_Guides/Setup_Rule_Action_Types#Setup_Actions

Depending on the complexity of the formula, you can do the calculation or aggregation or pivot right in the Bionic Rule and then write the transformed data directly to the field, too. Details on data transformation tasks: https://support.gainsight.com/Rules_Engine/Admin_Guides/Bionic_Rules_Task_Creation#Transform_Data