The next round
You can restart the Growkit
The Growkit is designed to use time after time again. It's not a disposable product.
The real kitchen farmer removes some of the mycelium [the white stuff] out of the Growkit, empties the remains and used that to restart the Growkits.
If that is not possible for one reason or another [when you have a contamination with another fungi] or if you are not a kitchen farmer, you can order a small bag of spawn @ RotterZwam.