'80 GS1100 Jumps Out of 2nd Gear

[Baatfam]

My own recommendation:
Purchase a used complete transmission, including the shifter drum and shift forks. Have the gears undercut.
Then pull the engine, turn it upside down on your work bench, split the cases, swap everything out, and reassemble.
One weekend's work if you do it in this order. Did it just 2 years ago on my bike.

Exactly what I did with my 1100E, only I did not get the gears undercut. I just inspected the used trans, found it had decent dogs and dropped it in. Swapping the trans is no big deal once you get the engine out...