I also have a similar experience, can't detect GPU and they say my Motherboard faulty, can't be fixed better to buy a new one.

But I wasn't satisfied with their service so I went to a specialist and they say it's not a Motherboard problem but it's the GPU. So he got it fixed and no problem now.

Never going to buy from this shitty company again. After-sales service is rubbish. They even charge for checking after the warranty period.