gear D does not work

Need more details. When, where, how, recent maintenance, what have you tried, what's still working, etc.

Yesterday It was good with a little vibration when standing at the traffic lights, but after about 10 hours ,during the time I drove the car suddenly stopped. I put the Gear in P and checked the car and everything was normal and when I came back to the car to start again but it did not move. (R - 2 - 1) works well . but ( D - 3) does not work like in (N) I don't remember when was last maintenance Saudi Arabia - Jeddah

Typical Allison & GM transmission limp mode. Assuming you're military, mentioning Saudi, Army trucks do the same thing. Some sensor failed, got unplugged, or harness problem. Have to crawl underneath.

I looked down the car and there were so many oils on the gearbox. Do you think this is the reason?

It might be low on fluid. Read the owner's manual for instructions on how to check and fill.

I have checked all the fluids and the sensors by computer and say all are ok.

Iam afraid it need rebuild

The mechanic decided it need to rebuild ... Thank you everyone