Taken from AUTHORS.data.txt in docs:
Tileset Spring: by Philipp Tscharntke
Taken from the README.data-license.txt in docs:
README for the MegaGlest Game Data
All the game data (including images and 3D models) are provided under the
Creative Commons "Attribution-ShareAlike 3.0 Unported" license. This license
lets others remix, tweak, and build upon your work even for commercial reasons,
as long as they credit you and license their new creations under identical
terms. This license is often compared to open source software licenses. All new
works based on yours will be bound to the same license, so any derivatives will
also allow commercial use. For more details about the MegaGlest game data see
the AUTHORS file as well as the copyright related files included in the docs folder.