I want to install a fortran binary from here GCC wiki GFortran binaries, but the binary for Mountain Lion is not yet available. Would the binary for Lion work, or would it be a terrible mistake to install it?
It should run just fine on Mountain Lion, and it should not do any damage to the Mountain Lion machine.
There is only one way to know for sure if it will work - installing it. I didn't want to do this because I am not sure how to test it, but will say that it shouldn't be a terrible mistake to install.
It will either not run (due to various things like Gatekeeper, minor API changes between Lion and Mountain Lion, etc) or not even install. Otherwise, a Mountain Lion specific build either uses new APIs, etc.
Finally, quickly browsing around online it looks like depending on your install there may be an issue
error trying to exec 'f951': execvp: No such file or directory". This site offers a fix, which looks like you need to uninstal the current Xcode, remove references to previous compilers like gcc, cpp, gfortran, and then reinstall Xcode, gfortran, and gcc.