Gases such as oxygen and carbon dioxide cross the plasma membrane by

Well, the cell membrane is full of a bunch of little holes. Sort of like a coffee filter.

What does a coffee filter do? Well, you put the coffee in it and run water through it. Then what?

The water is small enough to pass through the filter and bring some of the coffee with it. But the coffee grains are too big to pass through the filter. If we really wanted the coffee grains to get through the filter, we could force them through. Do you want chunks in your coffee? Eww. 

Now the cell membrane is the same way, the little things (water, carbon dioxide, oxygen, sugar) are all small enough to fit through the tiny holes in the cell membrane. This is called, "Passive Transport". Because the things can passively float their way in and out of the cell.

Now some other things are too big to fit through, so to make them fit, the cell has to use energy to open itself a little bit more to let them in. This is called, "Active Transport". Because the cell needs to be active and use energy to let those things in.

So to answer your question>>

The Gasses, such as Oxygen and Carbon Dioxide, cross the Cell Membrane(Plasma Membrane) by "Passive Transport".