AMD is still using an old microarchitecture called steamroller on the FM2/+ platform. This architecture is built on a 28Nm base, wich only allows a limited amount of transistors on the CPU die wich limits performance. The steamroller architecture also doesn't have 4 physical cores, it has 4 Logical cores and 2 physical cores, Windows still does detect it as a quad core cpu but in reality it isn't. This combined with a larger CPU die with less transistor and really only 2 cores and a core clock of 3.7Ghz makes it slower then the G3258 wich only has 2 cores but on a 22Nm Hasswell architecture wich allowes for more transistors wich increases performance. This combined with an unlocked multiplier wich allows it to OC all the way up to 4.6Ghz from 3.2Ghz (Keep in mind that you need a proper cooler) can make it crazy fast, note that the 860K also has an unlocked multiplier but isn't as friendly to OC as the G3258.
This is why I think the G3258 is faster, all my claims have all been tested and turned out to be true as I own both CPU's. So I can guarantee you that the G3258 is a little bit faster then the AMD 860K.