Gatsby would never go into hiding at this point in the novel, he has waited a long five years to get this close to Daisy once again. He has succeeded in his plan to be with Daisy, except everything goes very, very wrong in the hotel when Gatsby and Daisy confront Tom, her husband, with their love for each other.
Daisy is a fragile woman emotionally, Tom is an abusive husband and has emotionally abused Daisy with his open affairs and arrogant behavior. Gatsby feels very protective towards Daisy, he waits up all night long thinking that she might need his help. He is committed to her totally.
Daisy is the main reason that Gatsby does not go into hiding. He doesn't care if he is in trouble, in is going to take the blame for the accident any way. He is going to say he was driving to keep Daisy safe.
His love for Daisy is so totally consuming that he doesn't care about himself, he doesn't care what happens to him now. He has amassed wealth and power for one reason, to show Daisy what he has become, to impress her.
In "The Great Gatsby," Jay Gatsby is the protagonist. At the end of the story he and Daisy drive back from the city together. Daisy is driving the car when Myrtle is killed. Gatsby stands outside her house because he is worried about her.
The just before daylight Nick goes to the house to check on Jay and sees him dejected and tired. Nick says:
"You ought to go away,' I said. 'It's pretty certain they'll trace your car.'"
"Go away now, old sport?"
"Go to Atlantic city for a week, or up to Montreal."
"He wouldn't consider it. He couldn't possibly leave Daisy until he knew what she was going to do. He was clutching at some last hope and I couldn't bear to shake him free."
Gatsby doesn't know that his life is in danger. He decides to take a swim, saying that he hasn't used it all summer. He tells the only servant left that if a call should come while he is in the pool to bring it to him.
Meanwhile George Wilson is canvasing the county looking for the owner of the yellow car. Gatsby is totally unaware that Wilson is coming after him and he still hasn't given up hope that Daisy might call.
The great irony of your question is that Gatsby's entire existence is his illusion of his own reality. He truely believed that he could recreate his relationship with Daisy, and at that moment of recreation they could remain untouched by the progression of time. The reason why Gatsby does not go into hiding after Myrtle's death, or to add to your question report it to the authorities is because in order for his illusion or dream to remain part of his reality he must delude himself of the truth. Jay Gatsby's dreamlike reality gives him a sense of safety. As long as his illusion of Daisy's love for him remains intact he is incapable of realizing the violation of her actions as well as his inability to see her for what she really is.
why does nick say gatsby couldn't possibly leave daisy until he knew that what he was going to do. he was at some last hope and i couldn't bear to shake him free"?