A quick condition in gravel, esp dense gravel is unlikely. What are thr blow counts(the sum of the values for 6-12 &12-18 in. penetration)? I don't understand why it would be called as dense with low blow counts. If he were encountering drilling difficulties, that would probably be due to cobbles or boulders, which should be noted on the log. It sounds like more exploration is needed. I know everyone will say there is not enough time or money and we will deal with that in the field, but, when the contractor comes in with a big extra and needs a time extension, an extra boring or two will look cheap.
I agree with Big H and add that we would not leave the job until the borings were reviewed to see if there were any questions like this and if more borings were needed to resolve the questions.