Gravis left Manuel's faction after saying goodbye to everyone.
To alleviate their fears, Gravis told everyone that he would probably be gone for quite a while. Back then, the Heaven's Trial had taken Gravis over four years, which had been an incredibly long time for him.
Maybe this Heaven's Trial would also take an incredibly long time.
He also told them that, as long as his father didn't inform them, it meant that he was still alive.
Like that, they didn't have to worry about the unknown.
Gravis also said sorry that he couldn't be there for Stella's tribulation. Stella would undergo her tribulation in about 45,000 years, and Gravis assumed that the Heaven's Trial would take longer.
In fact, it wasn't only Stella undergoing her tribulation. Most others had become Star Gods just shortly after Stella, shortly meaning 100,000 or 200,000 years later.
Gravis referred to hundreds of thousands of years as shortly.