You can't dump the core of a 32-bit process with a 64-bit `gcore`. You still can't dump the core of a 32-bit process if a universal `gcore` is running in 64-bit; so, you have to use `arch -i386 gcore <pid>` to force it to run in 32-bit. Another option is to compile and install 2 binaries, `gcore32` and `gcore64`.