I am not sure if plants are able to use electrolytes as fuel in the game way that people do.

I would hypothesize that plants are capable of using some of the ions and definitely sugars in the power gel you are testing however since they use photosynthesis as a source of most of there usable fuel I would guess the physiology behind electrolyte use is likely different between plants and animals. Even the cells outer structures in plants are much more rigid and less permeable than our cells lipid membrane, which likely effects the plants ability to uptake some of these molecules.

As for the effect of the power gel on the lettuce sprouts length (diameter? they are very different measurements,) it is possible that the power gel could affect rate of growth in various ways. The ions in the power gel could be changing the osmotic balance between the plants cell and the environment effecting the ability of water uptake or in this case power gel uptake. If there was an effect on both diameter and length, it is also possible that the plant was getting more nutrients than it would just from water, and the plant did not grow as tall, but might have grown bigger around (diameter).

If the diameter was smaller than the control group, then the power gel may have effects on the turgor system of the stems, the ability to transport fluids from roots to stems.

These would all be very interesting things to investigate further. Have fun!