While Gel Stain can be applied over an existing finish, it was originally engineered for raw wood surfaces. We prefer applying the Gel Stain over a slip coat of mineral spirits instead of topcoat as shown in this video. (http://bit.ly/1WwDUQ1)
The slip coat will reduce the drag and help the stain "glide on" easier. The other important technique shown in this video is to apply the stain liberally in sections and remove the excess quickly. Gel Stain is so pigment rich you will get an uneven application if you try to dab it on with a dry rag.