We know the filter apps, adding a set of effects to a photo to gain a different visual impact. While the filters have simple names, they are actually applying a set of changes for the final effect. This can be seen when using a photoshop like app that breaks changes down to individual steps, each step can be changed before adding the next.
When using a film camera with lenses, one adjustment available is gels. These are most often recognized by color filters that are screwed onto the lens to take the photo though.
Gelo on the iPhone is those color lens gels, in an after effect electronic enhancement. Import a photo, then use the color spinner along the bottom of the Gelo screen to choose gradient or solid film and color.
Sliding the area at the top downward will provide access to color adjustment outside of the defaults in the application wheel.
If a gradient is chosen rather than a solid gel color, the filter start and end points can be adjusted easily via finger dots. Gelo allows the effect application to be at an angle too, not just up/down.
A different effect I have not seen in other apps is the ability to add a shape (circle, square or triangle) to a photo. The shapes can be solid fill, adjusted like the background gel filter effects.
The shapes can also be resized and moved around for impact. As well Gelo lets the shape to be changed if you find a square works better than a circle or triangle.