Valid keys in /etc/default/grub are as follows:
‘GRUB_DEFAULT’
The default menu entry. This may be a number, in which case it identifies the Nth entry in the generated menu counted from zero, or the title of a menu entry, or the special string ‘saved’. Using the title may be useful if you want to set a menu entry as the default even though there may be a variable number of entries before it.
‘GRUB_TIMEOUT’
Boot the default entry this many seconds after the menu is displayed, unless a key is pressed. The default is ‘5’. Set to ‘0’ to boot immediately without displaying the menu, or to ‘-1’ to wait indefinitely.