Can we use normal gel pens to write in zero gravity? Some say that astronauts use special space pens which uses pressurized nitrogen to write at space (zero gravity) and the normal gel pens does not write at space as there is no gravity for the gel to flow.
But, some others say that due to surface tension in the nip, the gel stays there and it is possible to write with it in space.
Which one is true? 
Can we use the gel pens we use everyday in space at zero gravity? 
 A: If pens required gravity to pull the ink to the tip, then you wouldn't be able to write upside down or sideways.  Perhaps you could perform an experiment?

If you were to do so, you would find that pens do indeed work (ish) upside down and sideways, and therefore do not require gravity for their operation.  However, they don't work particularly well (you have to shake them quite often), and there is more to writing in space than just the microgravity environment.  
Gel and felt-tip pens are optimized for use based on the viscosity of the gel/ink, which depends on the temperature and pressure of the environment in which they are being used.  Therefore, they aren't particularly dependable in the wide variety of conditions which exist in space, so a better alternative was designed.
So to answer your question, normal pens could be used, but specially designed "space pens" are better.
A: Well, I would say no. Gel pens have pigments suspended in gelled water based carrier. The tip has a very fine hole that allows this thick gel to flow if the tip contacts a solid surface and moves along it. For this to happen, it's necessary that the pen is tip side down (at Earth) so that the ink is attracted towards the tip and the pen can work. In microgravity conditions, though, it isn't possible for the ink to somehow get attracted towards the tip and hence, the ink flow is not facilitated. 
Pressurised space pens just fill in the role of gravity: they give the gel a tendency to flow through the tip if contacted by creating pressure inside the ink tubing. This explains why these pens can be used upside down as well! 
This is just my analysis of the case and might be found incompetent by more able fellows (I don't possess particular knowledge about fluid mechanics, but do have an intuition that allowed me to reason).
I hope this is of some help.
