Yeah, without an odometer reading there's no way to know how those gallons were used. If you still want to remember the details of your fuel-up you can add them as note. Just be sure that the next time you add a fuel-up, check the box next to "I missed a fuel-up" and you'll be set.
Another option is to estimate how much you drove after you fueled-up and subtract that amount from your current odometer reading. It won't be perfectly accurate, but sometimes if you haven't done much driving it can be easy to calculate.