When you write ./sage -i database_gap-4.4.12.pl.spkg, I think the character following the "p" (in ".pl.") is the letter "l", whereas it should be the number "1". Try it with a "1" instead of an "l" and see if that helps. (You can also avoid this issue by trying ./sage -i database_gap with no version information, and let Sage figure out what to download.)