For linux:(tried and checked in ubuntu)
To add SPL and APL syntax highlighting in gedit
Copy apl.xml and spl.xml into $HOME/.local/share/mime/packages/ (create if any of these doesnt exist)
Copy apl.lang and spl.lang into $HOME/.local/share/gtksourceview-3.0/language-specs/
Now open your terminal and cd into $HOME/.local/share/ and run the below command:
update-mime-database mime
Now close any gedit sessions and re-open.