Why Gorgias Talking About Nonexist ?

The explanation about “why” existent exist and contradiction with the non-existent.

In the text of Gorgias of Leontini; On The Nonexistent Described,

If existent is the same as the nonexistent, it is not possible for both to exist, they are not the same, both do not exist. To the which the conclusion follows that nothing exists. For if Neither the existent exist nor the nonexistent nor both, and if no additional possibility in conceivable, nothing exist.

Next it must be shown that even if anything exist, it is unknowable and incomprehensible to man. For, says Gorgias, if Considered thing in the mind are not existent, the existent is not considered. And that is logical. For if “white” were a possible attribute of what is considered, “being Considered” Also would have been a possible attribute of what is white; similarly, “if not to be existent” were a possible attribute of what is being considered, necessarily “not to be Considered” will be a possible attribute of what is existent.

Gorgias are Skepticism in view of epistemology, in characteristic of “exist” and “nonexist” there’s paradox divided to him. But, the rethoric in the text, make me confuse and after confuse you’re get the understanding.

I did not wrote the text with the argument or conclusion, but with new question.

How to understanding “exist” before “nonexist”?