Not much information in the video on how this was achieved, but a quick search for "gruv algorithmic music generation" returns the following: https://sites.google.com/site/anayebihomepage/cs224dfinalpro... . Extract:

We compare the performance of two different types of recurrent neural networks (RNNs) for the task of algorithmic music generation, with audio waveforms as input (as opposed to the standard MIDI). In particular, we focus on RNNs that have a sophisticated gating mechanism, namely, the Long Short-Term Memory (LSTM) network and the recently introduced Gated Recurrent Unit (GRU). Our results indicate that the generated outputs of the LSTM network were significantly more musically plausible than those of the GRU.