Depends on where the skimmer is positioned. The output plumbing CANNOT and I repeat CANNOT be going upwards not even by 1 degree. The skimmer returns water to the sump via gravity so if the plumbing goes upwards even by 1 degree, it will raise the water level inside the skimmer and therefore nullify the gate valve. So it does not matter if you just elbow into the sump or if you have to drill the sump, the plumbing must not be directed upwards at any point.
Also, make sure that the skimmer is higher than the sump. Otherwise, if the skimmer is at the same level of the sump, the water level in the skimmer will be dictated by the water level in the sump and NOT by the skimmer's gate valve and that is not recommended.