I want to show UTC or GMT

Showing Coordinated Universal Time or Greenwhich Mean Time is easy. Choose Pane > Clocks to open the Clocks pane and click the New Clock button.

To display time zones in the list, move the mouse pointer over the column headings, so that the word City highlights and a small white pop-up arrow appears. Click City in the column heading and select Time Zone in the displayed pop-up menu.

Select Coordinated Universal Time or Greenwhich Mean Time in the list, choose the desired format and click OK to add the new clock.