I need to pass the Type Name field timeline Gainsight data to SFDC.

I think you can add any fields on the Gainsight Timeline entry to your Activity Sync, so long as there is a SFDC field already created to map with.

I've done this by going to Gainsight Admin > Activities and Timeline > Activity Sync tab. The Field Mapping section has several default mappings. You can click + Add Field at the bottom to add new mappings, specifying your Gainsight and Salesforce fields to unite. In your scenario, I think you would select Activity Type on the Gainsight side, and you would need a Salesforce field in your destination object (SFDC Tasks or SFDC Events) with which to map.

Thanks Matthew - that's it. New problem is that I can't find my custom SFDC fields in the sync drop down.

fyi - I solved this problem. The SFDC picklist fields had a restriction of data that is permitted. Unchecking that solved the problem.