1. xemacs -vanilla
2. Options->Customize->Emacs->Environment-Gutter->Buffers Tab->
Buffers Tab Enabled
3. Toggle to nil, do Set
or do: (setq gutter-buffers-tab-enabled nil)
--> Notice that the buffers tabs are still shown.
If this option is saved and Emacs restarted, the change takes effect.
But setting it through the UI should have an immediate effect.