What do you mean by lost all gears? As in it will only shift into 5th and is blocked in the other gears or wont come out of 5th. There are 2 T60 bolts that go through the side of the case that are pivot pins for the 5th gear fork. Any chance one of them is loose or gone? If you pull the shifter off you should get a good view of the guts of the shift rails.
Mine broke a bunch of the dog teeth off the 3/4 syncro collar.
I haven't been inside a G-56 but 5th gear is 1:1 ratio, which means it goes straight through and doesn't use the counter rotating shaft. Sounds like the counter rotating shaft is broken and won't transfer power in 1st through 4th. Sound like it needs to come out for inspection.
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