How to choose GUID with High Sierra's Disk Utility?
In Disk Utility, near the close/minimize/fullscreen buttons, you can see a "View" mode dropdown menu. Set the view mode to "Show All Devices", then you'll see the "root" drives. Click on a root drive, and then click on the "Erase" button. Now an option to change the "Scheme" from Master Boot Record to GUID appears in the Erase subwindow.