The screen shot above shows the gummiboot help menu in the lower center of my laptop display. These are the key bindings:
- d (lower case) sets the currently highlighted menu entry as the default boot OS (or UEFI application) on boot.
- h (lower case) shows the help menu in the bottom part of the display. Just like shown in the screen shot above.
- t (lower case) increments the timeout to execute the default boot menu.
- T (upper case) decrements the timeout to execute the default boot menu.
- p (lower case) prints "something?". I haven't test what "print" exactly means here.
Therefore, to change the default OS/UEFI application to be automatically executed on timeout, you just need to select the menu entry you want and then press d in gummiboot. The newly set default menu should be effective immediately and preserved upon reboot/shutdown. Anyway, this post is further elaboration from: Modifying Gummiboot Configuration.