Gensym (Generate Symbols) is an old library for generating unique symbols (atoms). Such symbols are generated from a base atom which gets a sequence number appended. Of course there is no guarantee that `catch22' is not an already defined atom and therefore one must be aware these atoms are only unique in an isolated context.
The SWI-Prolog gensym library is thread-safe. The sequence numbers are global over all threads and therefore generated atoms are unique over all threads.