I’ve heard that the cores of sub-brown dwarfs (sub brown dwarfs mass objects that form the same way as stars and brown stars instead of forming like planets do.) are different from that of gas giants. Are the cores of sub brown dwarfs different then?
The most prevalent idea for the formation of gas giant planets is the "core accretion model". This proposes that the massive gas envelope accumulates around a core of icy or even rocky material. The presence of such a core has been established in Saturn.
Low mass brown dwarfs "that form the same way as stars" would not have a rocky/icy core and so this would be a definite difference.