How can I apply for PSUs through Gate?

You are going on a right track, or you are following a nice strategy too. But before you start your Mock Papers, just keep in your mind that the paper should be attempted properly without giving any pause, and after completion of test always consider the analysis report properly and find out the weaker areas and work on the same in a good manner. Try to avoid silly mistakes and do not attempt the questions in hurry. Good Luck!!!