Project Gemini is a LuthorCorp project dedicated to human cloning, accelerated growth, and memory implantation.
It is unclear whether the project was focused on cloning humans in general, or specifically on cloning Julian Luthor. Project Gemini may also be the continuation of cloning work from Project Ares, an earlier LuthorCorp project to create and clone super soldiers for the US Government.
An early model Julian clone named Adrian was a mixed success; the cloning and memory implantation worked properly, but the clone aged prematurely and didn't hold together well. Adrian escaped from Project Gemini, and threatened to expose Lex Luthor. Lex shot and killed Adrian.
A later model Julian clone named Grant Gabriel seemed to overcome those earlier technical difficulties. Lex considers Grant his brother.
Lex previously cloned Lana Lang as part of Project Ares, continuing Lex's pattern of cloning people that are an important part of his life. It was known as Model 503 which later was blown up when a car bomb went off in Lana's car allowing Lana to fake her death.