The President might issue orders (directives) to executive agencies to do a better job of carrying out the laws passed by Congress with respect to gun control, but he cannot initiate any sort of new law via an executive order. For example, he could order the ATF not to enable the arming of Mexican drug lords in the U.S. If he did that, it would be a good thing. On the other hand, if he exceeds his authority, the Congress and the Courts should accuse him of abuse of office.