First, the only Fiero to be built with the Getrag was the V6 from mid 1986 to 1988. The only Fiero built with the Isuzu 5 speed was the 4 cylinder cars from '85-'88.

There were also three 4 speed manual transmissions offered in Fieros, two with the 4 cylinder in 1984 and one with the V6 in '85 and part of '86.

Here is a listing of the transmission (including final drive ratio) and when it was offered (I also included the G6 six speed in this table for reference):

Muncie MY8 4 speed (economy) - 1984 - 3.32 FDR
Muncie M19 4 speed (performance) - 1984 - 4.10 FDR
Muncie M17 4 speed (Only with V6) - '85/'86 - 3.65 FDR
Isuzu MT2 5 speed (4 cylinder only) - '85/'88 - 3.35 FDR
Getrag MG2 5 spd (Only with V6) - '86/'88 - 3.61 FDR
G6 F40 6 speed (with the hipo Ecotech) - 3.55 FDR

All these trannys have the same bolt pattern, input shaft, and clutch (with the exception of the 6 speed, it shares the same bolt pattern, but the input shaft and clutch are different). So in order to swap to another type tranny - exc 6 speed - all you need to do is change the tranny mounts, stab and bolt the new trans in, and change the shift cables.

There are a number of reasons why you would see a V6 with an Isuzu. The biggest reason is that a Getrag can command up to $1K for a good one, while the Isuzu will give a lower RPM at cruise and a faster top end for just a couple hundred dollars total. Or perhaps it was a 4 cyl car originally and had a 2.8L V6 installed. Another thought is that the owner intentionally matched the transmission/gearing to the car's purpose. Faster acceleration, better mileage, higher top end, or a more even gear spacing (the Getrag has a huge RPM drop from 2nd to 3rd - the Isuzu is evenly spaced, for example).

I also have had a V6 with the M19 four speed in my past - - that's how the Cadero was set up before the 4.9 was installed in 1999. And I gotta tell you - - the M19 tranny woke that V6 up!! It was like adding 50 horsepower . . .

Hope this helps