I thought the reason the gladius was short was that it allowed the legionaries to get up close and personal with the enemy. The Gauls used longswords. That gets you a couple of good pokes at the Roman's shield before the Roman is in your face. You don't have room to use your longsword, while he's getting stabby. Same goes for spears: very effective until the enemy is up close, then useless.

One of the worst Roman military disasters was an expedition across the Rhine. A couple of legions got ambushed and wiped out at the Battle of the Teutoburg Forest (https://en.wikipedia.org/wiki/Battle_of_the_Teutoburg_Forest).