When describing something as "grimly mundane," it implies a bleak and dull experience. However, there are other synonyms that can be used to describe such a situation. Instead of "grimly," words such as "bleakly," "gloomily," or "sombrely" can be used. As for "mundane," it can be replaced by words such as "tedious," "boring," or "routine." These synonyms convey the same idea but with different nuances. For instance, using the word "tedious" implies a tiresome and repetitive experience, while "routine" means something commonplace and predictable. By using different synonyms, one can vary the language and style of their writing while conveying the same meaning.