pike-like fish, 1765, American English, shortening of garfish (mid-15c.), from fish (n.) + Middle English gare, gore "a spear," from Old English gar "spear," from Proto-Germanic *gaisa- "spear" (source also of Old Norse geirr "spear; point of an anvil," Old Saxon, Old High German ger, German Ger "spear"), from PIE *ghaiso- "a stick, spear" (see goad (n.)). The fish so called for its long sharp snout. Compare Edgar, garlic.
<a href="https://www.etymonline.com/word/Gertrude">Etymology of Gertrude by etymonline</a>
Harper, D. (n.d.). Etymology of Gertrude. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Gertrude