'gish' was introduced in the original Fiend Folio as a 4th level fighter 4th level magic user (in a little table, so there was no descriptive text beyond that). The term was generalized, including, sadly, by some of the wotcs themselves.
Obviously the F/MU concept did not originate with the Githyanki in D&D. It originated with the elf.
Yes, I voted 1, never heard Gish applied to Fighter/Mages until the Wotc Boards, needless to say I think it is superbly lame to use the name of a specific order (they also have antipaladins) of CE Astral pirates to describe any Fighter/Mage type, as you say, the original "Gish" is the Basic "Elf", so we might as well call any Fighter/Mage type "Elf". more legacy.