If you are looking for a digital version of (or Leyendas Perdidas in Spanish), it is a graphic novel by series creator Alex Hirsch that contains four original stories. Where to Find it
While digital previews or archive copies sometimes appear on sites like Internet Archive download+gravity+falls+leyendas+perdidas+nove+new
Note: Only do this if you have antivirus software and trust the source. If you are looking for a digital version