From Gnash Project Wiki
OpenBSD provides gnash packages via ftp. OpenBSD 4.1 has gnash 0.7.2; OpenBSD-current and 4.2 have 0.8.0. Setting PKG_PATH to an FTP mirror and running 'sudo pkg_add gnash' should be enough to install the binary package, which uses ffmpeg for media, agg for rendering, SDL for sound and the gtk toolkit. klash is not yet supported but should appear in the 0.8.1 port update.
Information on using the package system maybe found in the OpenBSD FAQ at http://www.openbsd.org/faq/faq15.html#PkgMgmt
The OpenBSD gnash port is known to work on i386, amd64, macppc and zaurus. More testing on other platforms would be appreciated.