Full Idea
I. Nothing exists. a) Not-Being does not exist. b) Being does not exist as everlasting, as created, as both, as One, or as Many. II. If anything does exist, it is incomprehensible. III. If existence is comprehensible, it is incommunicable.
Gist of Idea
Not-Being obviously doesn't exist, and the five modes of Being are all impossible
Source
report of Gorgias (fragments/reports [c.443 BCE], B03) by Diogenes Laertius - Lives of Eminent Philosophers 09
Book Reference
'Ancilla to the Pre-Socratic Philosophers', ed/tr. Freeman,Kathleen [Harvard 1957], p.129
A Reaction
[Also Sextus Empiricus, Against Logicians I.65-] For Part I he works through all the possible modes of being he can think of, and explains why none of them are possible. It is worth remembering that Gorgias loved rhetoric, not philosophy!