Eu não estou acostumado com o Gentoo (somente Ubuntu). Mas em /etc/grub.d/10_linux
a configuração é feita, usando variáveis definidas em /etc/default/grub
(os caminhos podem ser diferentes no Gentoo). Veja man grub-mkconfig
ou info -f grub -n 'Simple configuration'
. Este último diz:
The file '/etc/default/grub' controls the operation of
'grub-mkconfig'. It is sourced by a shell script, and so must be valid
POSIX shell input; normally, it will just be a sequence of 'KEY=value'
lines...
'GRUB_DISABLE_SUBMENU'
Normally, 'grub-mkconfig' will generate top level menu entry for
the kernel with highest version number and put all other found
kernels or alternative menu entries for recovery mode in submenu.
For entries returned by 'os-prober' first entry will be put on top
level and all others in submenu. If this option is set to 'y',
flat menu with all entries on top level will be generated instead.
Changing this option will require changing existing values of
'GRUB_DEFAULT', 'fallback' (*note fallback::) and 'default' (*note
default::) environment variables as well as saved default entry
using 'grub-set-default' and value used with 'grub-reboot'.
Portanto, procure por /etc/default/grub
(ou onde quer que seja) e defina GRUB_DISABLE_SUBMENU=y
. Em seguida, execute novamente ogrub-mkconfig
.