The 2mm charging spec has a current and voltage window of acceptable electrical supplies. At 6V the maximum current is somewhere around 1000mA. That 6V battery you linked probably consists of 4 cylindrical D or F size alkaline or heavy-duty batteries. It's capable of providing far more than that. So no, I wouldn't use that.

If you get two of them you can get 12V out of them, and use a car charger. Those I've tested even work when supply voltage drops below 12V, the better ones are still working at 6V input, so it would extract most of the juice out of the alkaline/heavyduty based batteries too (when connected in series to start off at 12V).

Just a guess, but two of those non-alkaline batteries you posted could probably when combined with a reasonable efficient car charger power GPS+display use for 50 hours.

You might also be interested in devices such as the Tekkeon MP1550 which can charge your N810 from 4 AA batteries. Maybe 4-8 hours of power per set of 4 AA NIMH. Would be less with alkalines.