Reduce the gel's size as suggested by FrankT, and also, use a brightness/contrast filter node in the function editor to get a pure black and white gel. Grey values don't show much in the light. I know this is not easy for a beginner, but do as follow:
Choose your gel, Vue will open the light gel tab of the light editor, right click the gel preview, and click the link between the gel and the altitude input. The link should now be thicker, then click the filter icon on the left toolbar to add a filter. In the drop down list of available filters, choose, brightness/contrast, then add contrast.
In some cases, there are laready some filters between the gel function and the altitude output, just add a new one, and don't touch the ones already there.
I don't find the "ready made" gels very good, and rely on this software to create the caustics patterns:
http://www.dualheights.se/caustics/