I got this from GOG and it launches through GOG galaxy. I found 2 large collections of user-generated maps but installation instructions were vague. The maps show up in my maplist.ini but when I launch the game I only see the standard ones available. I'm only playing local LAN and bot matches right now.

If the game doesn't show the added maps, this means the game reads an unmodified MapList.ini. Check your privileges for the XIII directory (your Windows account needs direct write/read permission) or launch the game as admin (cheap method).