Can you differentiate between Kc and Kp in this problem?
The equation yields Kp. 

$$\Delta G^{o}=-RTln(K_p)$$

There is, however, a relationship betweek Kp and Kc. 

$$K_p = K_c(RT)^{\Delta n} $$

From the balanced equation there are 3 moles of gas on the right (2+1) and 1 mole of gas on the left. In chem, delta means final minus initial and we arrive at $$\Delta n = 3-1=2$$

and you now can find Kc. Note that you should use 0.08206 L•atm/mol•K for R.



Sources: 


Zumdhal example 13.5
https://www.quora.com/In-a-gas-equilibrium-which-K-constant-is-used-in-the-equation-%CE%94G-RTlnK-Kp-or-Kc