Hi Manjit,
U can use Gel pens in the exams but that should be ANTI FADING. As far as I know ths comes in with the UNIBALL Gell Pens only. I am not marketing these pens but it is the fact that only in case of these pens ink does not used to fade.
In case of other gel pens ink fades as the quality of paper provided in the exams is not so good.
If u use a pen whose ink has a tendency to fade, in such a case ur answersheet will look very bad and clumsy typres as matter written on the either sides would overlap each other.
So either use Uniball or a good quality Ball Pen.
One more thing use only that pen in the Exams with which u had practiced ur preparations.
Hope ur query had got solved.