Could run hotter with the Slo-Syns due to a higher supplied current or voltage, or both. I didn't see the voltage rating in that .pdf either.
To get the best performance from Geckos, you usually want to run about 20x the motors rated voltage. With a 4.7V motor, you're only supplying about 5x the rated voltage. A 60-70V supply would probably work much better. Any higher and you could risk damaging the Geckos. Usually 2-3V motors are your best bet with Geckos.