Blayzify :
I have a 4.3 OC on my i5-6600k and the voltage is set to 1.25
should I have it set to constant or adaptive
While overclocking go with a manual voltage. Turn off Turbo, SpeedStepping (EIST) and C-states (energy savers). Find the highest multiplier that is stable with the least voltage. Once you find this (your max OC), re-enable EIST and any c-states you wish.
You can leave the manual voltage set, or you could try using an adaptive or offset voltage (Vcore) but note this may a bit tricky to get working on the Gigabyte boards.
This thread has some info on my adventures to get adaptive voltage to work on the Gigabyte board: http://www.tomshardware.com/forum/id-2970985/safe-vcore-6600k-tips.html#19199313. ZEBuckeye81 and myself were able to get it to work but philipew wasn't able to get it to work. It can be a challenge and you need to find your max multiplier and manual voltage first.