Hello roshni.gupta,
The gear button is the ‘menuButton’ data element. It looks like you have a lot of customizations on your UI so it’s a little difficult to figure out how you removed it. Usually to disable a button like this, we would use instance.UI.disableElement('menuButton'), in which case the way to re-enable it would be to remove this line of code or use instance.UI.enableElement('menuButton'). Otherwise, you might need to look through your code to find which customizations to the UI are removing/replacing the Header items where the menu button is located.
Best Regards,
Carlo Mendoza
Software Developer
PDFTron Systems, Inc.
www.pdftron.com