Unlike most other placoderm orders, such as the Antiarchs, or the Arthrodires, Gemuendina and its four other known relatives (or seven, if the three species of the ichthyolith genus Ohioaspis are confirmed to be rhenanids) had armor made up of a mosaic of unfused bony plates and scales. Because their armor was so fragile, few intact examples of rhenanids have survived in the fossil record. Because several regions of the Hunsruck lagerstätte were anoxic, thus free of scavenging organisms, intact, nearly pristine (albeit flattened) specimens of G. stuertzi have been found as a result. Also unlike other placoderms, it did not have the characteristic tooth plates of placoderms. Instead, it had star-shaped tubercle scales that allowed it to seize, then swallow fish and other animals that swam too close with its mouth.
Specimens of G. stuertzi ranged in size from 30 to 100 centimeters in length. In 1971, Erik Stensiö described a metre long specimen as a new species, "Broilina heroldi," which was later synonymized with Gemuendina.