Over on his OUP blog the Russian-born etymologist and author Anatoly Liberman speculates at length on the origin of the Anglo-American word gremlin and comes up with a somewhat surprising source:
“Our last demon for today is the gremlin. The noun has been around only since 1941 and is one of the war words that stayed and made a spectacular career. Still later (1970) the car called “Gremlin” was introduced. I have no idea why it was given such a mischievous name and whether it lived up to it. John Moore, in an article published in The Observer for November 8, 1942, discussed the folklore of gremlins and suggested that the sprite had come out of Fremlin beer bottles. This amusing explanation has been recycled by several authors, among others by Joseph T. Shipley, the least reliable etymologist among those whose books have been published by the otherwise dependable presses. The rhyme fremlin / kremlin / gremlin is obvious, but, regrettably, that is where we should stop, for we have no evidence that the word, which sprang up among British aviators, owes anything to its look-alikes. A more certain clue is the suffix, for it may have been borrowed from goblin.
Spitzer said in passing that gremlin had the same origin as grimalkin. Other scholars traced this word to an Old English or a rare Dutch verb. Such attempts should be rejected out of hand. If it is true that gremlin had not been heard of before 1941, in what limbo did it vegetate for centuries? It does happen that a word sometimes leads a hidden life in the language of the underworld, escapes from its environment, stops being slang, and enters aristocratic parlors. Gremlin does not seem to be one of such words. The suggestion that the etymon of gremlin is Irish Gaelic gruamin “ill-humored little fellow” is acceptable, but, as ill luck would have it, we don’t know whether the originator of the word was an Irishman or someone fluent in Irish Gaelic. Also, in the life of a word, its history following the moment of “conception” is of no small importance. How did gremlin gain such popularity? Why among pilots? Genies occasionally come out of the bottle; gremlins probably don’t. The origin of gremlin remains unknown, but a respectable imp should have a name beginning with gr-. Otherwise, who will be afraid of it?”
I know that the Irish word gruaimín does mean “gloomy, sullen little fellow”, being derived from related adjectives for gloom or despondency (c.f. gruamaire “gloomy, morose person” and the diminutive gruamachán). However it is difficult to see how this was reflected into English as gremlin. A more regular adaptation would surely have been something like grumeen or grumon. A quick check of eDIL doesn’t reveal much, even using Old or Middle Irish spelling. Neither does Foclóir Uí Dhuinnín. So I’ll have to have a deeper root around in my digital and hardcopy dictionaries. Meanwhile there is more information here.