Yes, it happens occasionally, at least to Brevas & the V7 Classic (ask me how I know).
If the gears change OK, but each shift requires priming first, then it's likely the return spring.
The issue has been treated before on the interweb - probably here, definitely on the MG_750 yahoo site, as well as a couple of other places. Can't put my finger on the links at present, but I did find a video on correctly installing the spring.
https://www.youtube.com/watch?v=rT6VAm0jJiYThe spring itself is cheap, the special tool not so much, so best try to borrow one.
The bike is probably best separated from the engine - easier than dropping one on the frame rails - and the "screw of doom" - the top bolt of the gearbox is located behind the bracket that holds the two throttle bodies. I was quoted the best part of a day's labour by an experienced guzzi mechanic - so it's likely to be about 3 plus for me, especially since there's a few other things to do while I'm in there.