I can imagine how it could be done in a dumb way. Are there any quick and scalable ways to produce line drawings with this gradient effect?
If your artwork is made with only strokes or only fills (you could just outline all your strokes if you have fills too), then you can either create a compound path (select all and cmd+8 or Object → Compound Path → Make) and apply a gradient as either stroke or fill:
Or you can group your artwork and add the gradient to the entire group, or simply select an entire layer and add the gradient to an entire layer using the Appearance panel:
If your artwork is made up of both strokes and paths then it may be hard to get the continuous gradient to match over both...
Another option is to use your artwork as an opacity mask on a solid gradient.
- Group your artwork and make it all white.
- Place it on top of a solid gradient.
- With your gradient and artwork selected hit "Make Mask" in the Transparency panel.
You would need to use the pen tool to draw them and then yes, use a stroke with gradient. There used to be some sort of auto trace function years ago in illustrator, but I think it changed names.