No, they're binary because they're compiled :P I meant binary as in "not scripting language". The actual plugin code is open, but the plugins have to be compiled for specific architectures. You can compile a plugin for i386, or for AMD64, or for PowerPC, but you can't run an AMD64 plugin on i386, an i386 plugin on PowerPC, etc.
Most of the plugins in Gens/GS should compile for multiple architectures. Some have optimizations on i386, which are disabled on non-i386 platforms. A few plugins (2xSaI, Super Eagle, Super 2xSaI, hq?x) are i386 asm only, and will not compile on other platforms.