To me, I wouldn't be too concerned, however, I would either unplug solar panel or throw a towel over it (cover completely). Open & close gate a few times with just battery not being charged. The higher voltage apparently is because the charge exceeds the intermittent gate motor load. Even cloudy panel is still charging some.
Just looking at pictures to me it's a nice set-up but maybe a little overkill...the solar panel & battery exceeds use time cycles.
Me, as an electronic technician, I would probably try a temporary experiment like a 12 volt landscape light or two, especially with a photocell (dusk/dawn). That gives a 1 or 2 amp load on battery.
Most likely it's designed the way it is because some applications the gate may be used every 5 minutes!
I have a landscaping friend who changes gate batteries a lot and thinking about it, it's probably from overcharging.
I think you'd certainly have higher voltage readings on your truck battery if it was on a charger 9 hours a day.