We have built Gardnerfish by modifying Stockfish the strongest open source chess engine in order to make it play Gardner Chess. In order to simplify our task we have considered b2 as the lower left square. Thus the initial FEN position is
"8/8/1rnbqk2/1ppppp2/8/1PPPPP2/1RNBQK2/8 w - - 0 1"
Starting from this position it behaves like a standard UCI engine. The
user has to take care of the fact that pawn promotions (which take
place on the second and sixth ranks) are sometimes badly handled by
chess interfaces (but internally it is well handled in
Gardnerfish). We advise the use of our 5x5 chess GUI
based on ScidvsPC.
Note also that Gardnerfish allows you to play any 5x5 chess variant as long as they do not include castling and double pawn moves. It can be used to study different starting position/setting like Baby chess, Jacobs-Meivoritz chess and Mallett for instance (starting/setting postion for these variants may be found on the wikipedia Mini chess page).
Latest sources are available on Github at https://github.com/MehdiMhalla/Stockfish. Some executables for various environments.