Rock Lee is very strong and he's been trained by Maito Guy but can he open the 8th gate now in Boruto?
According to this link:
By the age of 17, Lee was able to open six gates, and later as an adult in the anime, he was able to open the seventh gate.
Now to open it:
The Gate of Death (死門, Shimon), located at the heart, requires the user to stab their chest with their thumb to direct the chakra to the tenketsu
Now if Lee can open 7 gate then he can stab his heart to open 8th gate. So yes it is possible to open it unless there is an exception which has been not told yet but with given information available we can say that it is possible.