So I've contacted Acer Customer Services today with the question which one of those laptops is better. The staff at Acer wasn't helpful at all as he was avoiding the conversation and just sent me the links to datasheet 
So I decided to come here as from my own experience I know that community is always more helpful than any other customer service or the support. So back to the laptop itself.
As for the hardware, I know that those are virtually identical but still, there are some differences and I would want to make a good choice.
Predator 17 G9-793-775G has touchpad lighting, stripes and logo on the lid are backlit but those are the features that I don't care about at all.
But I do have some concerns about G-Sync as the G5 one does not have it (but some YouTuber actually says it has) would any of you guys have Predator 17 G5-793-77A2 and can confirm it? If it doesn't have G-Synch how it will impact my gaming and I must say a mainly play 3rd person open world games like GTA V, Mafia 3, Witcher 3 (hope this laptop can handle an hour or two of constant play). The other games I play, are simulators like CitiesSkylines (CPU intensive games yes I hope the laptop won't melt), Farming Sim 17, etc.
The other thing is HDD and I know that G9 has 7200RPM and G5 has 5400RPM one. It is not very important either as no one seriously is using HDD anymore. I've got some SSD waiting and also have some brand new HDD 7200RPM so don't care about the drives either.
But what is my concern is the Power Supply. G5s max output is 180W and G9 has 230W and this is probably because the G9 has more demanding (faster) HDD, more leds and lights (kinda Christmas tree), also additional Frost Core fan or something/space for the optical drive.
And the price of course... Actually (which is weird) the Predator 17 G5-793-77A2 is more expensive as Predator 17 G9-793-775G cost £30 less on Amazon.UK and this really baffles me...
So maybe you guys can give me some advice on which one would be better to buy. I thank you all in advance for any valuable input.