If you added hops, which I'm assuming you did during the boil, it's generally not a good idea to extend the boil impromptu. Reason being, it will cause more extraction of bitterness from the hops and therefore could swing in the other direction towards being too bitter. You're better off adding DME or LME to the boil at the end of the boil to try and get the gravity up, if you feel the need to correct the issue. If you're too high, you can add water to dilute the OG down a bit.
I used to miss my gravity a ton when I was getting used to my new setup several years ago. I would generally just go with it, knowing no matter what I was making beer, and would make notes that helped me hone the system in even further. Even when things get off a fair bit, they typically don't taste horrible...just not quite ideal in most cases. Generally, I didn't let it bother me as I was learning and just pressed forward for better results next time.
Hope that helps!