I started a 'how-to' about adjusting a gradient on-canvas, but it got very terdious 
Unfortunately the Gimp 2.10 help still refers to the original editor rather than on-canvas.
So I think you could use a white-to-transparent gradient but with a couple of stops added to divide the canvas up left mostly white, right mostly transparent.
I will attach a gradient might / might not be useful. Goes in your user gradients folder. Unzip and put white-to-transparent.ggr in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\gradients.
Apply across the canvas, adjust stops and center-point sliders to suit. example: https://i.imgur.com/AiBCFLN.mp4
Unfortunately the Gimp 2.10 help still refers to the original editor rather than on-canvas.So I think you could use a white-to-transparent gradient but with a couple of stops added to divide the canvas up left mostly white, right mostly transparent.
I will attach a gradient might / might not be useful. Goes in your user gradients folder. Unzip and put white-to-transparent.ggr in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\gradients.
Apply across the canvas, adjust stops and center-point sliders to suit. example: https://i.imgur.com/AiBCFLN.mp4