The J lounge would be the International Garuda Indonesia Business Class lounge at CGK's new Terminal 3.
It's pretty good for a J lounge for the most part IMO, as with most of Garuda's (refreshed) lounges.
You can find pictures of the Garuda lounge in CGK here:
https://news.detik.com/berita/327094...-di-terminal-3
That's the domestic lounge, although there isn't really much noticeable difference between the two.