We know that Gin is dead because Rangiku is in mourning; because Rangiku spoke of Gin in the past tense; because Aizen hacked him to pieces and stabbed him through and there was no healer near-by to heal him; because even if someone like Orihime resurrected him, he would not get off scot-free because whatever his reasons, he did do some terrible, terrible, terrible things and did help Aizen in his conspiracy. So he has no place left in the manga, thus Kubo killed him off. Simple as that.
Because he died, plain and simple. He was grievously injured, had an arm torn off, could no longer speak, thought to himself that he could leave things to Ichigo, said he was leaving and then closed his eyes. Rangiku later said that he left nothing behind for her to remember him by.
So in summary, yes Gin is really dead. Please direct all further enquiries to Kubo Tite, care of Shonen Jump. Thank you.