Both, but they have different meanings. "Nobody get out of the car" is definitely an instruction. "We have been surrounded by lions. Nobody get out of the car" "Nobody gets out of the car" can also be an instruction in some fairly rare situations but it is in the present tense so a more normal use would e just as a statement of fact. "Nobody gets out of the car while they are surrounded by lions. It's far too dangerous!"

They are both ok, but they both serve a different purpose. 'Nobody get out of the car!' Would be a command. For example, 4 people are in a car and one person says; nobody get out of the car! In other words; Everybody stay in the car! 'Nobody gets out of the car!' Would be an observation. For example you just watched a car smash into a building. When you see nobody coming out of the car you can say; 'Nobody gets out of the car' or 'Nobody is getting out of the car'. Hope this helps!

Nobody get out of the car! => US English.