Likewise, a long can gap down too. Admittedly, shorting inherently bears more risk so you have to be very careful.
A stock going down can only go to zero so, after a while, your risk to the downside is limited. So, if you say, buy XYZ stock at $2 and buy 200 shares, the most you can lose is $400 and your upside is many multiples of that amount. Contrast that by selling short by borrowing say 500 shares of ABC stock which you short at $50. Your risk to the upside is not limited to $25,000 but, unlimited. If the stock gaps up to say $150, you have to buy and cover those 500 shares that you borrowed, come hell or high water. Say you get out at $150 x 500 = $75,000. So, you got a huge $50,000 loss for your troubles. If you have only $30,000 in your account, congratulations, you still owe your broker that $20,000 difference.