Graystripe is not dead yet, when he dies he will join StarClan.
While we don't know for sure whether Hollyleaf joins Starclan or not, there are three main theories on this topic. The first theory says that she does not join Starclan because of her actions just before she died. The second theory says that she will still join Starclan in spite of this. Another theory says that she is not dead at all, but is trapped beneath the tunnels.
Firestar escorted Bluestar to StarClan when she passed away. He was a close friend and former deputy of Bluestar, and he guided her to join the ranks of the ancestors in StarClan.
The Darkest Path.
Graystripe died of old age in The Broken Code series, having lived a long life as both a warrior and later as ThunderClan's deputy. He passed away peacefully, surrounded by his friends and family in StarClan.
It is not confirmed when, but it will happen in Omen of the Stars
In the warriors series, the cats that join Starclan are those who die and have followed the warrior code, and served their clan faithfully.
Mosskit did join Starclan as you will learn (if you haven't already) in Warriors Field Guide: Secrets of the clans. He died while Bluestar was bringing Mosskit, Stonekit, and Mistykit to their father: Oakheart of riverclan.
If the cats are good even if they don't believe in StarClan they go to StarClan, but the cats like the cats from BloodClan don't. I'm not sure about the cats that have never heard of StarClan, but are good. Even if some cats believe in StarClan, but they do some really bad things, then they go to the "Place of No Stars".
Yes, when Bone, who was killed by many apprentices, from BloodClan, killed him.
Firestar joins StarClan after his death in battle. StarClan is where the spirits of all good cats go after they die, and Firestar shows his loyalty and devotion to his Clan throughout his life, earning his place among the noble ancestors.
She died saving her Clan and the others from a pack of dogs by throwing herself into the gorge
no he is firestars best friend for gods sake so he doesn't die main characters best friends never die but graystripe does get stolen by twolegs before they leave the forest so it takes a while for him to get back so firestar thinks hes dead but leafpool does not see him in starclan