Mgclabs assume if Google releases a desktop mode as part of aosp GrapheneOS would support it
If it ends up in AOSP, GrapheneOS is highly likely to ship it. If it's proprietary and invasive, like Android Auto, probably not. If the GrapheneOS Google compatibility layer can make it work, maybe after a while.
There's probably no way to predict until Google starts shipping something. There might be suggestive signs in the AOSP repositories a little in advance, but maybe not.
Overall, "It's difficult to make predictions, especially about the future".