Q:

What is a Gremlin?

What is a Gremlin? Is there a conventional, standard-issue portrayal of Gremlins in fantasy settings, much like there is for Dwarves, Elves or Orcs?
What games include portrayals of Gremlins?

A:

No, there's no standard. (Arguably there's no set standard for dwarves, elves, or orcs either, but the D&D juggernaut says otherwise.)
Gremlins are small, destructive, and malicious. They break and disable complicated things in subtle or obvious ways. The most plausible account of their folklore origin is rather recent, dating to World War II. There's no standard-issue portrayal of them because there was never a standard understanding of them in their original folklore—they are nearly impossible to catch in the act, and so they are more defined by the damage they cause, leaving their actual nature vague.
In AD&D 2nd edition gremlins are flying things that pester their victims and are nearly impossible to hit. They act in groups, and are about a foot tall. They like stealing things and destroying property. Other than that Monster Manual entry, I haven't seen gremlins in an RPG.

A:

Gremlins originate in the RAF around between WWI and WWII.
The Wikipedia Entry notes this:

Origins
The word "gremlin" originated in Royal Air Force (RAF) aviators' slang in Malta, the Middle East and India, with the earliest recorded printed use being in a poem published in the journal Aeroplane, in Malta on April 10, 1929.

The typical depiction from that era is a small, horned, tan or brown skinned homind, sometimes with wings, often wearing aviator goggles and leather helmets (presumably stolen).
Disney typified the imagery in several shorts, and so did Warner Brothers.

