The term "gill-like" refers to anything that resembles the opening on the side of a fish's head, through which they breathe. There are several synonyms for this term, including "branchial," which specifically refers to gills or structures that resemble them. "Fishy" or "aquatic" are more general terms that can be used to describe anything related to fish or aquatic life. "Lung-like" is another synonym that can be used to describe structures that resemble gills and function similarly for oxygen exchange, such as those found in certain species of insects or amphibians. Ultimately, the synonym used will depend on the context and intended meaning of the description.