In the photos they show the tailgate being down with a can in the holder but they don't show how the rear window lifts up with a can in the holder.I would be hesitant to try and rig up 10 gallons of gas onto the Gobi ladder - two 5 gallon jerry cans will be around 70lbs when filled up. I don't doubt the ladder can support that weight (you want to climb up it after all!) but the stress from driving, especially rough driving off-road, might be too much for it over time. 5 gallons would hopefully be fine though, and you could probably bolt one of the military jerry can holders on the ladder without too much trouble.
Not sure if the 80 series has the same rear crossmember bolt pattern as our trucks, but this idea is pretty nifty:
Land Cruiser 80 FJ80 FZJ80 LX450 Jerry Can Holder Mount/Hi-Lift Jack Mount JM4 | eBay
Fits 1991-1997 Toyota Land Cruiser 80 Series FJ80 and FZJ80 and Lexus LX450 . JM4 Jerry Can Holder Mount & Hi-Lift Jack Mount. For use with LX450 you do have to drill holes in the plastic cover to access the.www.ebay.com