ScummVM game doesn't fit the screen

The Curse of Monkey Island from GOG installed via Lutris (Ubuntu 18.04) is not scaled up to fit the screen. This is alleviated by using "OpenGL" as the "Graphics Mode" for that game in the GUI, but the next time the game is launched from Lutris my graphics settings have been reset. I don’t know why.

I tried adding --gfx-mode=opengl as an option in lutris but ScummVM complained about unknown mode, and neither man nor the ScummVM wiki seem to be aware of the existence of the OpenGL graphics mode. Nor is Lutris’ ScummVM runner.

How is this supposed to be done without having to reconfigure ScummVM every single time?