The usual way to run GCC is to run the executable called
<machine>-gcc when cross-compiling, or
<machine>-gcc-<version> to run a version other than the one that
was installed last. Sometimes this is inconvenient, so GCC provides
options that will switch to another cross-compiler or version.
The value to use for machine is the same as was specified as the
machine type when configuring GCC as a cross-compiler. For
example, if a cross-compiler was configured with
i386v, meaning to compile for an 80386 running System V, then you
-b i386v to run that cross compiler.
2.0, meaning to run GCC version 2.0.
-b options work by running the
<machine>-gcc-<version> executable, so there's no real reason to
use them if you can just run that directly.