CAT is the entrance examination for admission to Management programmes in the 20 IIMs and as many as 1000 participating Institutions. For being eligible for CAT, you should have completed your Bachelor's degree in any discipline with a minimum aggregate of 50%. It is 50% for General Category and 45% for Reserved Categories. GAT is the entrance examination for admission to Engineering, Pharmacy and Architecture courses after 12th. So for GAT, you should have completed your 12th with Mathematics, Physics and Chemistry as mandatory subjects. In short CAT is for Master's programmes and GAT is for Bachelor's programmes.