At a basic level, journal entries can be used for any and all transactions in double entry accounting. But Manager provides many additional ways to enter transactions. These are really just shortcuts for frequent types of accounting events. The main thing is to understand that every time something happens, there must be offsetting credits and debits.
Read the documentation at this link first: http://www.manager.io/documentation/expense-claims
You have to set up an account for the employee. The fact that the company owes the employee for the fuel represents a liability. Note, that if the person who buys the gas is the owner, the purchase represents a contribution of capital and goes on the owner’s capital account. In that case, you probably would not actually reimburse the owner.
The expense is recorded as a debit to some expense account, such as Motor Vehicle Expenses. Exactly which one will depend on how you’ve arranged your chart of accounts.