GrapheneOS uses the AOSP Keyboard, which to my knowledge doesn't allow you to do this. Updating these AOSP apps that are already relatively out of date seems counterintuitive when the keyboard app could eventually be replacing with something that's a top-down improvement.
I know you said that you don't want to install a different keyboard, but that's why we install different apps; because the current ones lack functionality that we desire.
I would recommend looking into Florisboard. On the FOSS side, it's one of the best, if not the best option.