There is a way of adjusting the thickness of paint layer lineart provided that you don’t want to change the thickness by a large amount and that the following conditions are met:
- No sharp edges or sudden curves.
- No rentrant curves or cusps.
- No lines close to each other.
- The lineart is on a white layer, not a transparent layer.
- The resulting lineart will be black.
If you apply a Gaussian Blur (Filter -> Blur -> Gaussian Blur) to the lineart of about the same size as the width of the lineart and then use Filter -> Adjust -> Threshold and adjust the threshold point, you can make the lineart thicker or thinner. You will lose any fine details and there will be distortion of curves if you try to change it too much.
If you need to do this to existing simple lineart then it’s a possible solution.