When using the out-of-the-box (OOTB) option to disable end-user access to the Classic UI, the Gantt chart tab is no longer available within the Modern UX (MUX).
v16.3.3 and above
The Modern UX Timeline view serves as the alternative to the traditional Gantt chart. Since you cannot update baselines directly in the Timeline, you can perform baseline management by configuring the specific Baseline module within the project blueprint.
If you notice any missing features or have ideas for improvements, please submit an enhancement request.