/usr/share/bash-completion/completions/gsettings 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.
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[ -z "$BASH_VERSION" ] && return
####################################################################################################
__gsettings() {
  local choices coffset schemadir
  if [ ${COMP_CWORD} -gt 2 ]; then
      if [ ${COMP_WORDS[1]} = --schemadir ]; then
	  # this complexity is needed to perform correct tilde expansion
	  schemadir=$(eval "echo --schemadir ${COMP_WORDS[2]}")
	  coffset=2
      else
	  coffset=0
      fi
  else
      coffset=0
  fi
  case "$((${COMP_CWORD}-$coffset))" in
    1)
      choices=$'--schemadir\n--version\nhelp \nlist-schemas\nlist-relocatable-schemas\nlist-keys \nlist-children \nlist-recursively \nget \nrange \nset \nreset \nreset-recursively \nwritable \nmonitor'
      ;;
    2)
      case "${COMP_WORDS[$(($coffset+1))]}" in
	--schemadir)
	  COMPREPLY=($(compgen -o dirnames -- ${COMP_WORDS[${COMP_CWORD}]}))
	  return 0
	  ;;
        help)
          choices=$'list-schemas\nlist-relocatable-schemas\nlist-keys\nlist-children\nlist-recursively\nget\nrange\nset\nreset\nreset-recursively\nwritable\nmonitor'
          ;;
        list-keys|list-children|list-recursively|reset-recursively)
          choices="$(gsettings $schemadir list-schemas)"$'\n'"$(gsettings $schemadir list-relocatable-schemas | sed -e 's.$.:/.')"
          ;;
        get|range|set|reset|writable|monitor)
          choices="$(gsettings $schemadir list-schemas | sed -e 's.$. .')"$'\n'"$(gsettings $schemadir list-relocatable-schemas | sed -e 's.$.:/.')"
          ;;
      esac
      ;;
    3)
      case "${COMP_WORDS[$(($coffset+1))]}" in
        set)
          choices="$(gsettings $schemadir list-keys ${COMP_WORDS[$(($coffset+2))]} 2> /dev/null | sed -e 's.$. .')"
          ;;
        get|range|reset|writable|monitor)
          choices="$(gsettings $schemadir list-keys ${COMP_WORDS[$(($coffset+2))]} 2> /dev/null)"
          ;;
      esac
      ;;
    4)
      case "${COMP_WORDS[$(($coffset+2))]}" in
        set)
          range=($(gsettings $schemadir range ${COMP_WORDS[$(($coffset+2))]} ${COMP_WORDS[$(($coffset+3))]} 2> /dev/null))
          case "${range[0]}" in
            enum)
              unset range[0]
             ;;
            *)
              unset range
             ;;
          esac
          local IFS=$'\n'
          choices="${range[*]}"
          ;;
      esac
      ;;
  esac
  local IFS=$'\n'
  COMPREPLY=($(compgen -W "${choices}" -- "${COMP_WORDS[${COMP_CWORD}]}"))
}
####################################################################################################
complete -o nospace -F __gsettings gsettings
 |