The gdtools package is licensed under the GPL-3 (see the files LICENSE and
DESCRIPTION), except for the files listed below.

inst/fonts/stix/STIXTwoMath-Regular.otf
  Font "STIX Two Math", version 2.13 b171, unmodified file from
  https://github.com/stipub/stixfonts (tag v2.13b171,
  fonts/static_otf/STIXTwoMath-Regular.otf).
  Copyright 2001-2021 The STIX Fonts Project Authors
  (https://github.com/stipub/stixfonts), with Reserved Font Name "TM Math".
  STIX Fonts(TM) is a trademark of The Institute of Electrical and
  Electronics Engineers, Inc.
  Licensed under the SIL Open Font License, Version 1.1: see the file
  inst/fonts/stix/OFL.txt.