I could not find this in the GNU Chess manual by searching, although I might be overlooking it, under a different name.
What I am seeking to do with GNU Chess is quite simple: Specifying the position of every single piece on the board, for both players, and deciding who starts after that. I'm currently reading a beginner's book on Chess, as I know nearly nothing about it, and would like to follow the tutorials, which suggest to start with particular positions with a chess computer or advanced player, and figure out how to survive.
The optimal feature would be having the possibility to keep those starting positions in text files, so that I can reuse them and modify them.