I understand the battlegrounds maps have been released on steam workshop. Is there a site where gog customers can download these maps as well?

While I own both the Steam and GOG versions of Battlezone 98, Steam provides me with a better platform for distributed development across my three-or-so PCs. As such, it's the Steam version of the game I'm updating these maps with.

Steam Workshop allows developers to update their mods piecemeal, and pushes those updates automatically to the end-user. GOG does not allow this, and as such it is better for maps to receive a single release; if a new version of the map were released for every update made on the Steam Workshop, there's bound to be somebody who ends up on an old version without realising.

The versions of the Battle Grounds maps currently on the Steam Workshop are unfinished, and by quite a long way. They've been released to allow me to gauge interest and get the maps publicly tested (which is more than some of them got for their original release). The GOG release will include only the finished maps for the reasons outlined above - this is a drawback of GOG's inferior mod support, I'm afraid.

In short, Battle Grounds is coming to GOG - just not until it's finished. :)