Does copyright restrict the use of GNU Smalltalk?
It does, but in general you don't need to worry much about the licensing of your Smalltalk programs.
The class library is licensed under the GNU LGPL. Thus, it permits using the library together with non-free programs.
The virtual machine and the bindings to external libraries are licensed under the GNU GPL, with a special exception that allows Smalltalk programs to be linked with the functions exported by the virtual machine and said bindings.