This snippet for zsh still has some rough edges but works for the majority of cases. Automatically extends any global alias when space is pressed in zsh. For ex. I have `alias -G G='rg -s'`, so if I type `command | G` it will autoexpand it to `command | rg -s` and so on.

  globalias() {
    local raw word
    # raw last blank-separated token, exactly as typed
    raw=${LBUFFER##\* }
    # shell-parsed last word
    word=${${(z)LBUFFER}[-1]}
    # if user typed \alias, don't expand
    if [[ $raw == \\* ]]; then
        zle self-insert
        return
    fi
    if alias -- ${(q)word} &>/dev/null; then
        zle _expand_alias
        zle expand-word
    fi
    zle self-insert

zle -N globalias bindkey ' ' globalias