The summary field should be in your line items table.
The report should be based on your line items table.
You can make a subsummary part on the report that is based on the transactions, so that when sorted, the summary field in line items will show the total for that transaction "group"
Then a trailing grand summary part at the end with the same summary field in it would show to total of all line items on the report.
One which table (occurrence) is your report layout based?
If based on your line item table, you should get the grand total that you want. This field won't give you a grand total on a layout based on transactions.
Note that you can sort/group your line items on a line items based layout with fields from transactions included in a sub summary part to show the same data as you have on your transactions layout, but with greater flexibility.
You can also define a calculation field in transactions that uses the sum function to report the total gallons for a given transaction. A summary field in transactions that totals this calculation field would also give you the needed grand total.