I've just spent some time playing around and have it figured out! I hope this helps you:
The G'MIC filter can now be found under Filter > Start G'MIC Qt. Go to Black and White > Colorize lineart [propagation].
In the Input/Output box on the left, you need to select "All" for the inpute layers, but that's all you need to do. The rest of the settings are on the right, which you can see follow the tutorial from the OP
Click apply and you're done!
To the OP: thank you so much for this, it's going to save me so much time.