(a) Use the download button at https://github.com/libav/gas-preprocessor
(b) Extract the archive
(c) Remove any other file by same name which you have downloaded and you were experimenting with.
(d) copy the file gas-preprocessor.pl at /usr/local/bin 或者/usr/bin/
(e) Set permission of the file to Read and write by all (777) if a -> d doesn’t work