It occurs to me that martial training in Spain for a Roman was likely superior to any other theater of warfare. The reason is the legacy left by Sertorius, the Marian rebel who, in the 70s, created a rump state complete with Senate in Spain. Sertorius raised three or four armies in Spain and trained natives in fighting Roman legions. He even outfitted native armies with heavy Roman arms and trained some to fight as any Roman legion would have.

So there may have been no better place for Caesar to gain experience as a commander. The "native" Spanish armies he fought were probably far worthier opponents than the Gaulish armies he defeated.