Gnash is a free browser plugin and stand-alone player meant to play Adobe Flash files. It is released under the GNU General Public License and it is a "high priority" FSF project. It loves to spend 100% of your CPU time doing mostly nothing. It does not even play flash video at big sites like YouTube and fails horribly at playing most other content. Gnash has not been updated since 2012.
It plays mostly nothing, specially not flash games. The homepage gnashdev.org is down and its bad care to still offer the installation by package manager. It is crap.
Browser plugin features
Newer version of gnash do play video files at a few video big sites such as YouTube just fine. It mostly fails to play video files embedded in blogs and other small sites. It mostly fails to play flash games.
100% CPU hogging
Gnash loves to use the CPU and it really loves to use all of it. It loves using CPU so much that it continues to do so long after you close the browser tab or window where a flash object was playing.
It appears that the opengl rendering is specially CPU-hogging and it makes gnash so slow that it is impossible to play videos close to smooth when opengl support is compiled in. It also seems that using the Anti-Grain Geometry (agg) rendering engine is a very good idea.
Ubuntu users have reported that gnash slows their systems down to the point where they become unusable. Ubuntu users may want to compile gnash themselves rather than use the official packages.
The Fedora Project insists on using the gstreamer backend due to some local patent laws in the fascist US regime. Distributions outside that corporate two-faced one-party-system police state appers to prefer using ffmpeg over gstreamer -- and the performance of ffmpeg is typically better.
Developer involvement indicated
Gnash development was initially headed by software developer John Gilmore. Is is still part of the project. Rob Savoye is the current project head.
There are no free software alternatives. All free software flash players are useless piles of dung. It's sad but true.