GNU Gnash is the GNU Flash movie player — Flash is an animation file format pioneered by Macromedia which continues to be supported by their successor company, Adobe.
The project seems to be no longer developed. Last version, 0.8.10, released in February 2012, can be still downloaded from Git.
Gnash is the GNU flash movie player, which can be run standalone on the desktop or an embedded device, as well as as a plugin for several browsers. Gnash is based on GameSWF, and supports most SWF v7 features and some SWF v8 and v9.
It might replace Adobe Flash player.
some SWF contains ActionScript 3 which is not yet supported by Gnash. The movie may not work as intended.