It is not true that the only UPC translator for Linux is the GNU one. Berkeley UPC also lists Linux as a supported platform (and I personally testify that it does support Linux).
Both translators (as current UPC compilers actually use an intermediate source-to-source translation) use different runtime libraries with GUPC also being able to use the Berkeley UPCR runtime instead. UPCR is more capable than the runtime that ships with GUPC in terms of transports supported and synchronisation primitives provided.