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.
you could try riding mower chains or do like some of us winter dirt bike riders do and screw in sheet metal screws or studs into the tires....................of course they are not that good for your asphalt driveway but great traction on gravel and ice
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