Do all of the molecules in a gas to have the same speed?

Question

When can this happen?

Answer 1

no they do not.

All the molecules of a gas are moving at different speeds as they are continually bouncing off eachother and flying around.
We measure the AVERAGE speed of the molecules of a gas as it's TEMPERATURE. That is in fact the deffinition of temperature.

Answer 2

No they don't have the same speed, but velocities will be distributed in a Normal distribution. If you drew a graph of velocity against number of molecules it would be a 'bell curve' centered around the average.
Imagine a few molecules are stopped. Other moving molecules would whack into them and they would soon be moving at a similar speed. Same thing for a few fast ones.
the important thing is the Average speed. This is directly related to temperature. The ideal gas laws work on the average speed. you cannot model many millions of tiny molecules all moving and colliding, so you have to use the average speed.