Dear Kamal, you cannot appear under 2 different categories unless since the time you applied for GATE, the sub category or caste you belong to, has been added into OBC Category from General Category. In such case you have to have proof of such documents. The other option is all the while you belonged to OBC but decided to opt out of any reservation but now want to reclaim it. In such case, you can go ahead and fill your category as OBC. Apart from these 2 conditions, you cannot change your Category.