/usr/share/bash-completion/completions/gdbus is in libglib2.0-bin 2.40.2-0ubuntu1.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32  | # Check for bash
[ -z "$BASH_VERSION" ] && return
####################################################################################################
__gdbus() {
    local IFS=$'\n'
    local cur=`_get_cword :`
    local suggestions=$(gdbus complete "${COMP_LINE}" ${COMP_POINT})
    COMPREPLY=($(compgen -W "$suggestions" -- "$cur"))
    # Remove colon-word prefix from COMPREPLY items
    case "$cur" in
        *:*)
            case "$COMP_WORDBREAKS" in
                *:*)
                    local colon_word=${cur%${cur##*:}}
                    local i=${#COMPREPLY[*]}
                    while [ $((--i)) -ge 0 ]; do
                        COMPREPLY[$i]=${COMPREPLY[$i]#"$colon_word"}
                    done
                    ;;
            esac
            ;;
    esac
}
####################################################################################################
complete -o nospace -F __gdbus gdbus
 |