Why you do not read?
Syntax is GRUB_GFXMODE=auto,1024x768x32 (sample)
# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `videoinfo'
GRUB_GFXMODE=auto
and check what is possible, not what you like with
you can see them in real GRUB with the command `videoinfo'