That's not entirely correct.
Only the G3258 (anniversary edition) processor can overclock, and that will overclock on non-z motherboards as well. Otherwise no celeron processor or i3's can overclock (except for a tiny bit which no one really cares about). And the G3258 processor isn't available in Pakistan yet. And even this processor, at around 4.5 GHZ matches an i3 only.
Celeron will cause quite a noticable bottleneck. With the r9 270, in moderate cpu demnading games,a very rough estimate is if you get 40 fps from i5, expect to get 30fps from the G3240. Results can be worse and better depending on the game. check out
this review for comparison of the G3220 (negligibly slower than your's), and
these benchmarks
But the most annoying part which I am quite sure will happen is that in some parts where graphics are important, the game will run smothly and then suddenly you do something that requires a bit of cpu and BAM, your game starts to lag like crazy. So although the average fps may be good, there will most likely be a big difference between the 'max' and 'min' fps you get.