~/.config/gtk-3.0/gtk.css:

/* Decrease the tabs bar height in gnome-terminal
 * See:
 * https://stackoverflow.com/questions/36869701/decrease-the-tabs-bar-height-in-gnome-terminal
 */

terminal-window notebook > header.top button {
  padding: 0 0 0 0;
  background-image: none;
  border: 0;
  margin-right: 10px;
}

terminal-window notebook > header.top > tabs > tab {
  margin: 0 0 0 0;
  padding: 0 0 0 0;
}

terminal-window notebook > header.top > tabs > tab label {
  padding: 0 0 0 0;
  margin: 0 0 0 0;
}

Before

After

From: https://stackoverflow.com/questions/36869701/decrease-the-tabs-bar-height-in-gnome-terminal