To my mind, there is no doubt that it's harmful to look at glossy screens for a long time. Though the words and picture on the glossy screens are much clearer than other screens, your eyes will feel much more tired when you use it. But there are also solutions to prevent it, such as washing your eyes with cold water after looking at it, and using some eye drops. In a word, I suggest you not look at glossy screens too long or change other kind of screens.
Personlly, I perfer matte screens rather than glossy screens, because glossy screen is bad for your eyes in the long run due to the glare from it. As we can feel that glossy screen makes the color and images sharper. That is the reason why it is harmful to our eyes. Generally speaking, glossy screen have more reflection and glare than matte screens. Frankly speaking,there are reports that suggest use of glossy screens could increase the risk of health issues. In my opinion, we had better think about when choosing a screen.
The glossy screens will give more clear pictures or videos. However if you stare at it for a long time with little rest, it will be bad for your eyes. Your eyes nerves will be intense. Your eyes will get tired easily. You should better have the suitable rest for the eyes when you look at the screen. If you feel uncomfortable at the eyes, you could just use some eye drops to release it.