Can a built-in grill sit on granite, or does it need to be in a stainless steel jacket?
The following quotes are from the 
Lion 90000 Series Instruction manual:

"Upper and ground level vents must be provided for combustion air on both sides of built-in cabinet."

"Upper and lower vents (20 sq in. minimum must be provided) for combustion air on both sides of built-in cabinet."

When instructions say must it is mandatory, not "nice to have".

The manual says the cabinet can be made from any non-combustible material, granite is stone and non-combustible, so it will be fine.