Actually the G is speced for premium yes, but in most parts of the country premium means 91 octane so it comes expecting 91.
The engine management system will pull reliably timing when run by 87 and I would expect to see no engine damage.
At least nothing thats in any way significant.
some studies have shown extra wear particles in oil change analyses in cars run on 87 vs 91.
So I dont blame u for being hesitant..
I owned a 2012 Pathfinder in the past, exact same engine/transmission, as the G35, which also specs premium and I ran mid range, 89, in it most of the time since the price difference to premium was so stark.
What altitude is the car at?
The further above sea level the lower the octane requirement for a given vehicle.
If the rest of the deal is good and u found what u were looking for I would probably still pull the trigger.
Then again, it might be indicative of the car not being owned by someone who loved it.. so other maintenance may have been neglected also..