Your scenario is a little convoluted. What does any of that stuff have to do with paying for the gas you use in the loaner?!?
But yes, I would understand that I purchased a computer from a place that is far from my home and I would understand that I need to bring it to them for repair during the times they are open for business. It honestly would never occur to me that would be an issue.
For one, the dealership is open on the weekend. I'm sure if you told them you could only drop the car off on Saturday and pick it up the following saturday they would do that and leave you in the loaner for the week...my dealer would do that. I've dropped my car off and its needed warranty repairs and they have told me to take the loaner on a trip.
Also, like I said if you explained the situation to the dealer I bet they would have come and picked it up for you and dropped off the loaner.
Sometimes all you have to do is ask. Did you even talk to the dealer to see ways in which they could accomodate you?
In any event, if you didn't have time to stop for 5 minutes and get gas, just let the dealer charge you for the gas. Maybe they would, maybe they wouldn't. Or maybe next time you should buy a car with a dealer thats closer to your home if this is such an imposition.