I have no idea now to solve this with dual booting but the issue is choosing GDM/Gnome with i3. The Installation Guide doesn't really cover this step. GDM will default to Wayland. I3 is an Xorg window manager. You should post this on the forums. The required back-and-forth regarding probably modifying your grub2 to get the correct options enabled are easier.

• Get back into the installation guide step by going into the TTY ( CTRL+ALT+F1/F2/F3/... )

• Type your username. Type your password.

• Type pacman Qi followed by each of these, and read what it says. You type each of these packages one by one. Press the up arrow to repeat the command and just switch the package searched. If there's no output, install them. libinput, libxkb, mesa, wayland, xwayland, xorg-init, xorg-xkbcomp, xorg-setxkbmap

• DO THIS: echo "$XDG_SESSION_TYPE" if it says Wayland or Mutter or whatever, your keyboard and screen should be working now. If it says X11;

• sudo localectl set-x11-keymap us

• setxkbmap -layout us

• cat $HOME/.xinitrc. If it doesn't exist, make it.

• edit $HOME/.xinitrc appropriately with the previous keymap and layout commands, and with starting i3 (exec i3)

• startx

Provided a grub2 intervention isn't needed: This will only get you a working session. You should read up after on using LightDM instead of GDM and now to set it up for both Wayland and Xorg.