So much depends on condition. Have a look through the For Sale posts on the forum, on Ebay (a "decent usable" one just went for £1000) and just Google 'Gilera Nordwest for sale'.
If a good runner in very nice condition with a fresh MoT I'd guess it could sell for around £1500/2000. Values for bikes advertised in Europe seem similar.
www.theparking-motorcycle.eu/used-motorc...gilera-nordwest.html
At the end of the day, for insurance purposes they will only pay out based on recent sales. As a > 15 years old 'classic' you might try for an agreed value figure, but realistically Nordwests haven't seriosly become collectors items yet. Now the early ones are VMCC eligible that might change.
"Do not adjust your mind, there is a fault in the reality"
1957 175 Sport