Last night my gm decided to spring a rule on us stating you can't have magic items whose enhancement bonus equivalent is greater than character level/3 rounded up. He claims this rule is from the game masters guide and further claims it's a rule Paizo carried over from 3.5. But I cannot for the life of me find it anywhere. Does this rule actually come from the GMG? If so what page? Is it from 3.5 and if so what book and page? Or does this sound like a house rule?
Your GM either created a house rule or is improperly interpreting the Magic Item Creation rules.
Based on the rules here:
Creating magic armor has a special prerequisite: The creator's caster level must be at least three times the enhancement bonus of the armor.
The Weapons section has the same statement. So you can see that this only applies to creating magic items, not using them. Player Character Wealth by Level (how much funds you have) is supposed to regulate how quickly you gain powerful items, but if you come into a small fortune somehow, you are more than able (via base rules) to buy and use a +5 Vorpal weapon at level 1.
Well in theory you should have much more powerful magic items then that anyways. But yes that is almost definitely a house rule.