Lukaku has been dropping hints that he is ready to leave Goodison Park in recent times, with many his latest interviews causing frustration with Toffees supporters.
The 23-year-old appears to have his heart set on a move, and Gray believes that the Belgian international will move on.
However, Gray does feel that there is a stumbling block in the transfer, due to how much Everton will demand for the prolific front man.
The Toffees are under no pressure to sell Lukaku, following the investment of Farhad Moshiri, and they are expected to demand a huge fee if they are to allow Lukaku to leave.
“Lukaku’s gone. He’s definitely out the door, it’s just a matter of when and where,” Gray said.
“The only thing that will be a stumbling block for Lukaku, will be how much. How much is somebody prepared to be for the young man.”
Lukaku’s departure from Everton would be a major blow for the Merseyside club, as he is their main source of goals.
Lukaku netted 25 times for the Toffees this past campaign, with his reputation being enhanced even further.
If Lukaku does leave Everton, then the Toffees are likely to be in the market for a forward, with Sandro Ramirez the latest to be linked (Daily Mail).