1. Start with the image you want to add the gloss efect to. I am using this simple signature.
2. Then, add a new layer and select the rectangle tool and then click the fill button which is circled in the image. Then draw a white shape in the new layer. I used a simple square. you can also do waves, circles or whatever you want. to do a wave, select the line tool and draw a striaght line in the middle of the image or whever you want to gloss to be. then drag the little boxes to "mutate" the line into a wave. select the bottom part of the wave then hit Ctrl+I to invert the selection. then fill selection by using the square tool.
3. Double click on the layer with your white shape on it and decrese the opacity until your satified with the results. I used an Opacity level of 64, but feel free to do whatever you like.
your results should look something like this.
