>Looks very much like a format that should just have been gzipped JSON.

For application file formats that require storing binary blob data such as images, bitmaps, etc , many in the industry have settled on "SQLite db as a file format": (https://www.sqlite.org/appfileformat.html)

Examples include Mozilla Firefox using sqlite db for favicons, Apple iOS using sqlite to store camera photos, Kodi media player uses sqlite for binary metadata, Microsoft Visual C++ IDE stores source code browsing data in sqlite, etc.

Sqlite db would usually be a better choice rather than binary blobs encoded as Base64 and being stuffed into json.gzip files. One of the areas where the less efficient gzipped JSON might be better than Sqlite is web-server-to-web-browser data transfers because the client's Javascript engine has builtin gzip decompress and JSON manipulation functions.