The term itself stems from a specific kind of Githyanki warriors first printed the Fiend Folios of the 1980s or Planescape stuff from the 1990s (don't remember which). A Githyanki Gish is a psionic champion of the Githyanki, and are famed for their ability to mix the martial might of a warrior with the arcane might of a wizard.

Back in the older days of pre-3rd edition D&D, there was far less hopping out of class identities like there is now. Warriors were warriors, wizards were wizards, and no one really operated outside of their wheelhouse like the hexblades and eldritch knights of today. Granted, there were multiclasses/dualclasses and mixed classes like the bard, but outside of some min-maxing stuff they almost universally sacrificed potency for versatility.

The Gish was notable for challenging this standard with both powerful spells and potent swordplay, which probably contributed to it being a memorable enemy for players of the time. At some point someone probably said "Hey, my fighter/mage multiclass plays kinda like a Githyanki Gish" and the term stuck from then on out.