At the moment, I either use the brush or just make a massive vignette and drag it to one side of the image. It'd work just like the vignette control, but be flat instead of round, perhaps with positive and negative values on the sliders to get it either bright or dark and maybe with the ability to add color as well. Such a simple little device would save me so much time.

Head over to Snapseed, and you won't find a graduated filter.It seems odd. Seriously, go and have a look! We've got a vignette with inner and outer brightness controls (something that even Lightroom Classic doesn't do), a brush with exposure controls, and even the ability to edit each stack and selectively paint the scene wherever we want it, but there's no graduated filter. I know it might take some time and money to program this, but hey, it's Google. Instead then, why not give us the option to reorder the tools? We could click and hold, then drag them into a different tile arrangement. Okay, so alphabetizing doesn't appeal? Sure, I understand. Also, I've been using the app for so long that I know the names of the tools I want to apply but can't necessarily remember where they are on the screen, so I have to go hunting for them. How often do you add vignettes to photos? More commonly than applying a grungy filter, I bet, and yet it's all the way down the bottom of the list. It sounds weird to put everything in alphabetical order, because what if the tools you use most often are at the bottom of the list? I mean, that's why it's laid out like that, so the most commonly used tools are at the top, ready to go. Stop! I can already hear you muttering at me, and yes, I know Lightroom doesn't do this either.

Snapseed's Tools panel is currently not in alphabetical order and would do better if it was customizable.

Photo editing snapseed and lightroom