It's an NX pet equip that allows you to automatically buff yourself whenever the buff's expired. It will come in handy when using the buffs such as Combo Attack. The best class I can confirm are Crusader, DrK (Dragon Knight), and Priest.
Now here are the skills I confirm to work with the auto buff.
Knights of Cygnus
= not confirmed
Note: Auto Buff Gem doesn't work with potential skills such as Decent Hyper Body and Decent Sharp Eyes.
As for Party Skills, the pet only buffs when the skill expires, so if the person you're going to buff is out of range, the person doesn't get the buff and won't if the buff is in effect. Which means you have to go towards the person that didn't get buffed and right click on the skill to rebuff so that he/she gets the buff.
I'll add in more buff skills if anyone could confirm the skills for Hero, DarK, Bishop, and any other classes. That would be greatly appreciated.
For those of you wondering why I'm using different names, I'm used to the original KMS names or I'm not always used to GMS names.
For Dual Blade and Cannoneer skills, please check the Legends skill list, that'll help you.
Credit goes to @Pink Bean and @Dark Link for confirmation on other skills.