Yep, those small hard rubber tires don't do well on gravel, but they are even worse in snow, and slide around VERY easily on wet pavement.

For something to work on gravel or off a smooth hard surface, you really need one of these lift trucks with the large tractor tires. The good news is many of the bigger off road lift tractors have a slant 6 in them too.

Probably need to send it away and get something useful for your intended purposes.