I really recommend Fenix or Python to start developing games for the Gp2x. Great titles as Camelot Warriors, Ruckman, Boomshine or Protozoa were witen in Fenix.
Learning C could be really hard if you do not have a computering background and some experience in programming. If memory and speed are really a problem in your game, then learn C and hard code during several weeks. But use Fenix or Python to develop simple yet funny games during a raining evening!