Add a gzip compressed gallery-catpkg.json.gz alternative asset

[amis92]

File	Size	Ratio
bsdata.catpkg-gallery.json	17.1 KB	1.0
bsdata.catpkg-gallery.json.gz	2.52 KB	0.15

This is a major bandwidth saving (85% less). Of course, we'll always provide original JSON as well.