Tom, is that a trials bike, I assume?
https://www.retrotrials.com/gasgas-model-history.html
When GasGas was still owned by GG and then Torrot, most if not all of the manuals were on line at the Company website. Now that KTM has bought the Company, the links to manuals for the old bikes are gone.
I would call John Shirt in the UK. I also think that Jim Snell at trialsparts USA will know, if anyone does. Finally, you can get help and info. at Trialscentral - a good forum for all things trials related (and UK/European based). You might post up your question there.
Hope you have success!
Jeff
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https://johnshirtmotorcycles.com/
http://www.trialspartsusa.com/
https://www.trialscentral.com/forums/