Well, I have only got a few small bits missing - so I guess I'm worth a fortune. Definitely not a GS, in fact I am surprised to see the covers aren't the more rounded type given the year.
Looks like a 150 Sprint to me. I had a '74 Sprint Veloce when I lived in London. Should have kept it, it was bloody good fun when it wasn't wearing out the little cruciform gear thingy. Wore out one side of the tyres, was a funny little thing but kinda cool.
I can fix a cruciform in 45mins. no more jumping outta 2nd. Practise. It may be a crossover model. Between gs and next model as the frame looks later to me. But so much is interchangeable across all models it honestly could be anything. Including a bitsa. Check frame and engine nos.
Ive done so many of them I refined it down to 45mins timed. I used to them for all my mates and associates back in the day. Jumping outta 2nd constantly was the very first symptom. All I need is the new cruciform and a crankcase gasket. As long as the long 13" crankcase bolt isnt seized in, which many of them are.
If I could get a suitably ratty / original one... my last one was an import from Rome, something like this would be lovely.
My last was a 1970 Lambretta GP250 AF Rayspeed Super X type. But altered to stage 6 Ts1 225. Two guys from Nuremburg came over for it in 1991. Sold for £900. Value today? 10k+ dammit.