The current release of Gnash is 0.8.9. You can install it with your package manager, or download the source. The source can be found at a GNU FTP Mirror. First, choose a mirror near you, then select the 'gnash' directory. Once you have downloaded the latest revision, follow the installation instructions in the documentation.
Gnash 0.8.9 is also available via Git:
The latest development sources are available via anonymous Git: