You can invert the Linear Gradient Mask, which basically flips the mask coverage. It keeps the non-destructive nature, meaning that you can readjust the inverted Gradient Mask by moving it, dragging the start line or the end line to new positions, or rotate it.
- Select the layer with the Linear Gradient Mask in the Layers tool
- Right-click and select Invert Mask or choose Layer -> Invert Mask.
- You can now readjust position, coverage, and rotation is you see fit.