Need help inputting a transaction.

[Bruno]

Here is my question: A friend of mine lent me $20. I created a new 
account, under liabilities, called "Name_of_my_friend" and gave it a 
positive value of $20, so I knew I owed him $20.

On the next weekend we were going to make a trip (using my car) to a 
close city. We decided to split the costs of gas and he would have to 
pay me $20, so he said: "You owe me $20, so my part is already paid".

Then I withdrew $20 from my checking account and paid for the gas. Now I 
don't know how to input this transaction, because I want to:

Bring the "Name_of_my_friend" account to zero, since I paid him.
Add $20 to the "expenses:auto:gas" account.
Decrease my "checking account" account by $20, since I withdrew $20.

Is it possible to do that in a single transaction, if it is, how so?

Thanks for your help.