Most shops put GL5 in as that's what most manual transmissions (at least those that do not require ATF) use.
Unfortunately, the sulfur in most GL5 gear oils is not compatible with some of the materials Nissan uses in their transmissions - like the synchros, etc., etc. It is very easy to make the mistake, especially if the person changing fluids doesn't RTFM or know about Nissan manuals. This is a well known idiot-generated problem for Nissans; all manual Nissans not using a bought-in transmission like the T-5 are incompatible with most GL5 lubricants.
More info here:
http://www.nissanforums.com/b14-95-99-chassis/104196-gear-oil-gl-4-vs-gl.html
Worse than just being simply the 'wrong' additive, the sulfur actually destroys Nissan synchros (which are made of bronze and copper) through the process of chemical etching, and then your transmission is toast. I had the very last Nissan model which
could use GL5 in the transmission, and that was the T-5NWC equipped 1985 300ZX Turbo. The next year the only manual transmission choice was the then-new in house five speed, and an unwary or inattentive tech installing GL5 in the 1986 transmission could wreck it in no time flat. I think there actually was a Nissan TSB about it at the time.
I remember one Nissan club 'work day' where everyone else started screaming about the gear oil when I pulled it out of the hatch - which stopped when I reminded them that I had a T-5 and could actually use GL5 safely.
When I think of 'wrong' additive, I think about someone putting a non-LSD gear oil into an LSD and poisoning it (there were some that were incompatible with clutch type LSDs a while back - they 'poisoned' or contaminated the clutch discs and rendered your LSD into an open diff.) However, it didn't acutally cause mechanical damage. This is actually worse than just 'wrong.'
Edit: Putting GL5 in a Nissan transmission is about the only thing that will kill a stock Nissan manual at stock power levels, unlike my former T-5. Unfortunately, Blind, it looks like you got bit by an idiot grease monkey. Whoever changed your transmission lube last owes you a transmission; a simple and inexpensive oil analysis can prove that the oil is in fact GL5.