- Use setxkbmap to remap the key (does not require a daemon and is independent of your desktop environment or window manager). Don't forget to add the command before the exec gnome-session (or similar) line in your ~/.xinitrc or ~/.xsession:
setxkbmap -option caps:escape
setxkbmap can be found in package extra/xorg-setxkbmap.
dconf-editor → org.gnome.desktop.input-sources.xkb-options → Add caps:escape to the aforementioned field.
gnome-session-settings → Startup Programs → Add → Name=Remap caps lock to escape, command=setxkbmap -option caps:escape
FYI, I obtained the XKB rule by grepping /usr/share/X11/xkb/rules for caps and esc.