One of the benefits of using gels and drinks on race day is that you are able to determine which supplements (or even brand of supplement) work best with your body. Some gels are more dense than others, and this can be a cause of GI distress. If you introduce these things only on race day your body may disagree with them which could lead to a bad race and possibly a need for a new pair of bibs. Of course that's an extreme case...but I always believe the best way to be prepared for a race is to mimic the conditions of that race.