Code:
sudo systemctl set-default graphical
should reset boot mode back to graphical, assuming while in multi-user mode you didn't damage something. From the login screen there is an option to choose the session type, which normally remains set to whatever was logged into last.
"initramfs unpacking failed: decoding failed" is not something related to your graphical session type. What brought that message to your attention? Were you examining dmesg output, or did you see it in journalctl? What, if anything, have you tried to do about it?
We need to know what you can do before suggesting how to proceed, or what might be wrong. Are you still able to reach multi-user and a login prompt? If yes, instead of startx, try:
Code:
sudo systemctl isolate graphical
If you don't want a Gnome session to be available, you could remove it while in multi-user:
Code:
sudo apt remove gnome-session
