There are several synonyms that can be used to describe something that is "grimly predictable". One of them is "foreseeable", which means that something can be easily predicted or expected to happen. Another synonym is "inevitable", which implies that something is going to happen no matter what. "Routine" is another synonym that can be used, indicating that something is done in a regular, predictable way. "Monotonous" is another option, which suggests a lack of variety or excitement. Additionally, "repetitive" is a synonym which can be applied to something that is routinely or predictably done again and again.