Try using LaTeX's jobname variable instead of Scribus's $scribus_file…

…name$

contrib/900_gregorio.xml

@@ -45,14 +45,14 @@
 		\pagestyle{empty}
     	\setlength{\textwidth}{$scribus_realwidth$ pt}
 
-        \begin{filecontents*}[overwrite]{$scribus_file$-score.gabc}
+        \begin{filecontents*}[overwrite]{\jobname-score.gabc}
 </preamble> <!-- this line's indentation is purposefully off because of the way that scribus interprets the preamble section -->
     <postamble>
         \end{filecontents*}
 
         \begin{document}
         $scribus_greconf$
-        \gregorioscore{scribus_file-score}
+        \gregorioscore{\jobname-score.gabc}
         \end{document}
     </postamble>
     <tab type="settings">