Ungradable opposition refers to words that cannot have varying levels of intensity or degree. Therefore, finding synonyms for such words is a challenging task. One possible alternative is to use the term "absolute opposition." Another well-known synonym for ungradable opposition is "binary opposition," which is commonly used in fields such as linguistics, philosophy, and anthropology. Other synonyms that can be used include "divergent opposition," "polar opposition," or simply "opposition." However, it's important to note that these alternatives may not always capture the same nuances of meaning as the original term, so it's best to use them in context-sensitive situations.