I looked up the Gladius rules again, and as it turns out, the “1/3 Rule” applies to ALL abilities, not just spellcasters.
So, if you are a barbarian that rages 14 rounds per day. You are limited to 1/3 that number, rounded up, in a given match. So you can only rage 5 rounds in a match.
(Same goes for 1/day abilities. So if you have smite evil once a day, 1/3 rounded up is the same number: ONE.)
If you have a rod of empower metamagic 3/day, you can only use it once in a battle.