Use an iframe to show only SharePoint Online Gantt

#suiteBarDelta, #s4-ribbonrow, #s4-titlerow, #sideNavBox, #DeltaFormDigest{
  display:none !important
}

#contentBox{
  margin:0
}
#contentRow{
  padding:0
}
#s4-workspace{
	height:auto;
	width:auto;
}

You can then use an iframe to reference your out-of-the-box SharePoint page.

<iframe src="path/to/calendar-with-css.aspx" />

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