Gel batteries are expensive, and require pretty precise control on charging. Peter_C can give more info on that part.
For a smallish motor, driving minimal electronics for not really long periods, battery selection is not that critical. Any starting battery should do fine. If you expect to run electronics for long periods while drifting or anchored, go with a dual purpose style (though there is some speculation they're not all that different.) Check the Interstate website; they have pretty good info (as I recall - I ended up with a couple of blue-top Optimas.) West Marine is selling Duracell batteries - they looked OK to me, but didn't come in the size I needed.
You probably won't get much benefit from a deep cycle battery unless you really run it down without the motor running.