last time i shot my ar15 i did not had a single issue, when i was cleaning i notice that one gas ring gap was bigger than the others, should i replace it to prevent issue?

http://i61.tinypic.com/1zodezc.jpg

Looks like a broken ring.  I'd order a set to be prepared, but I wouldn't replace it until you start having issues unless it's your HD weapon...

i wonder how it broke, i only shot maybe 500 to 600 rounds.

whos the manufacturer?

the ar is a diamondback db15, like i said i recently took it out to the range and every shot was consistence and i didnt get any failure, that ring i notice it like that while i was cleaning it.

I'd replace all three of the rings and keep an eye on the new set....if it happens again you need to really look into what's causing it, if they are good after 4-500 rounds though I'd figure the original set was just a dud.

I have seen gas rings destroyed in less than 700 rounds before...replace and drive on.

I would report it to the manufacturer with an email.  Then I would replace all three rings as rings should be replaced as a set, not one at a time.  I would see if this bolt repeated this behavior.  If it repeated it I would ask for customer service on the item.  A trade out on the bcg.  You should get thousands of rounds out of a gas ring set.  You setup will perform with only one gas ring, it is a redundant system.  And the gaps have no need to be staggered.  It will function perfectly if the ring gaps are all lined up.   I would, and do, keep several sets of gas rings on hand.  Gas rings are an item you want plenty of just in case.  I don't know about guns I have gotten rid of but I have never had to replace any rings on any AR's I own.
 

Did you lube before shooting? And you shouldn't have had an issue as long as the gaps aren't aligned. But replace ASAP and grab yourself an extra QUALITY bolt. These things happen.

Total myth gas ring alignment has zero bearing on performance.  Lube is an excellent point though.

Gap is 100% myth but lube is not you must lubricate the rings just like the rings in your car engine.
Replace rings and shoot on.

You should only have an issue if all 3 rings fail.  You really only need one for a proper seal, the other two are just there as failsafes.  In anycase, it's a cheap and easy replace so you may as well do it and see if the problem persists.