I had not noticed the lack of an audio in. I had assumed it had one. Even my MicroGranny has one.
They say that when the €50,000 is reached they will add “Sampling via USB interface/dongle. Record and save samples on the GR-1.” So you’ll attach an USB soundcard (even a 30 eur behringer uca-202) and you’ll be able to sample directly in the machine.

Strange choice though, especially for the price.