The result is a playable ROM that fits in a single ROM memory bank (16 KB) and contains both a trained NN and inference code. The NN running on the Game Boy achieves 93.86% accuracy on the MNIST test set; although not a good result for MNIST (since it is such an easy classification task), it's quite decent for a model running on a Game Boy.
You can find the compiled ROM on the Releases page (https://github.com/jongoiko/gb-nn/releases), feel free to try it out!