I had already installed and tried GTA V on my laptop but it seems to have trouble running it. After a few minutes of gameplay, it starts to lag and crash while the first few moments of gameplay are quite smooth. Recently I saw someone playing GTA V on the Lenovo G40-70 (my PC model) smoothly on YouTube. I don't know why I can't do so, so please help me.
I wouldn't call that playing smoothly. There are many models of the G40-70, you'll have to be more specific with the rest of your model number, or tell us the actual specs of the computer. CPU, GPU, RAM would be a start.