The forums belong to PC Specialist who build and sell custom PCs so it's not possible to advise in detail on other companies' systems.
But in general terms (and this advice is no secret as it's in plenty of topics already on these forums):
- make sure the monitor is plugged into the graphics card, not the motherboard
- check your game settings - a 1050 won't run gta on max plus lots of AA very well
- check CPU and GPU temps with software like Realtemp
I believe overclockers uk also have forums where you could ask about the kind.of performance.you should expect and there are forums independent of vendors eg Tom's hardware
If there is a problem with the PC there would usually be a warranty so do contact the vendor too to troubleshoot any issues
GTA 5 is one of the most demanding games out there at the moment, the graphics are incredibly realistic. I've even seen comparison pictures where you have to guess which screenshots are from GTA 5 and which ones are pictures from real life, and I was honestly fooled on a few of them - I thought some of the GTA 5 screenshots were real life pictures.
With your settings, honestly, I'd be expecting 30 FPS if you were playing it on medium settings with some of the more demanding settings turned off. You can see my most recent PC in my signature down below my post, and I get between 40-60 (usually holding around 50) with a mixture of medium/high settings.