Hi Marissa,
Greetings from Syncfusion.
After reviewing your query, Currently do not have support for timeline templates in Gantt functionality.
We already logged feature request for the requirement. You can track the status of the feature from the feedback link given below.
Feedback - Provide support for timeline template in JavaScript | Feedback Portal (syncfusion.com)
Note: To view the above feedback, kindly login into your account.
We have already lined up some major features, we could not implement this support immediately. We will implement this and include it in any of our upcoming releases. Please cast your vote on this feature. Based on the customer demand we will prioritize the features in our upcoming road map. You can communicate with us or track the status of the feature using the feedback link.
Regards,
Sridharan