but what is its purpose on gaming motherboards? cus i saw one motherboard (i think its asus or msi i dont remember) that has one.
Curiously, PCI isn't for gaming so that's actually the reason why it still appears on boards. You could even find ISA and EISA slots well into the PCI and PCIe slot transition. Why? again, because people and organizations have costly to replace legacy add-in cards that needed a home. That's doubtless a reason why VLB (Vesa Local Bus) and AGP died so quickly - they were never very useful for anything besides gaming graphics cards.
Like DSzymborski said, Gaming is just a name applied to a motherboard, along with the flashy heatsink designs, solder mask and LED bling, to add appeal. It's only purpose...