It should theoretically be possible. However, it would be easier and probably cheaper just to replace the engine with one designed for gasoline.
No, you cannot convert a gasoline engine to a diesel engine.
Simple answer: 100 pounds of Propane => 100 / 4.24 = 23.6 gal the "standard" LP cylinder for a gas grill is 20 pounds = 4.7 gallons The conversion for LP is Gallons x 4.24 = Pounds if one is trying to compare LP to gasoline, one must also consider that LP provides about 80% of the energy of gasoline. In a small portable generator, it is nearly transparent, but if one wishes to convert a car to LP one gets less mpg and hp.
A engine exchange will be necessary. Remove the diesel and install a gasoline type.
A gasoline engine converts chemical energy into mechanical energy.
The purpose of a gasoline car engine is to convert gasoline into motion so that your car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is an internal combustion engine -- combustion takes place internally. The engine turns the explosion that takes place into Horsepower & Torque. Torque gets the car moving and Horspower keeps it moving.
The "...convert a gas car to electric..." implies removal of the gasoline engine and replacing it with an electric motor, creating an all electric car. If on the other hand you intend to keep the gasoline engine and use it when the batteries are low, then you will have created a hybrid gasoline-electric vehicle. With that qualification, and the assumption that you meant a full conversion from gasoline to electric, then the answer to your question is that IF the gasoline engine is removed from service, then the computer "that controlled the gas engine and its components" is no longer necessary and can be removed also. Also, the portion of your question relating to the gasoline engine computer suggests your thinking that it could be reprogrammed somehow to control the electric motor. Since that computer is designed for and is dedicated to the gasoline engine, it cannot be be reprogrammed and used to control the electric motor(s).
No, a diesel engine will not burn LPG. It would destroy a diesel engine. A gasoline engine can, with some modifications, burn LPG.
a gasoline engine is an Internal combustion engine
It set for LP gas need to convert to natural gas
the engine is running
The gasoline engine came first. The gasoline engine was invented and patented in 1823 with the first efficient gasoline engine developed in 1876. The diesel was invented in 1893 which was 70 years after the gasoline engine.
I assume LPG means liquid propane gas. Few automobiles in the US are powered this by lp gas. Some large companies such as utility contractors use lp gas for their trucks, and many companies use lp gas for fork lifts and other such machinery. The expense of converting an engine from gasoline to lp, coupled with the relatively low cost and availability of gasoline (gas prices are rising, but compared to Europe prices here are very low indeed)provide very little motivation to power an automobile in this manner.