This was said by Vidal's maternal grandfather, Thomas Pryor Gore, as recalled by Vidal: "My grandfather, Senator Gore ('I never give advice') was suddenly Polonius; he also changed his usual line from 'Never have children, only grandchildren' to 'Be not fruitful, do not multiply.' " [Palimpsest, ch. 3: The Desire and the Successful Pursuit of the Whole]
Misattributed

“Each writer is born with a repertory company in his head.”

Gore Vidal

“By the time a man gets to be presidential material, he's been bought ten times over.”

Gore Vidal

“Never offend an enemy in a small way.”

Gore Vidal book Julian

Source: Julian

“Well, you have to work out what it is. They are a little splinter. They can't summon many voters at any given time. They are a minority of a minority of a minority. They have everybody buffaloed because the great corporations like them and pay money to their candidates for sheriff and senator. And they're playing big-time politics. Yes, indeed. But the average person doesn't like them. You know, any time I want to get applause — and I lecture across America in state after state after state — when I fear things are getting a little low, I always say, “And another thing: Let us tax all the religions,” I bring down the goddamn house with that. And any politician would if he had sense enough to do it. The people don't like their tax exemption.”

Gore Vidal

On the religious right in America http://therealnews.com/t/index.php?option=com_content&task=view&id=31&Itemid=74&jumival=342 <br class="br">2000s, What I've Learned (2008), Gore Vidal's America (2009)

“The average "educated" American has been made to believe that, somehow, the United States must lead the world even though hardly anyone has any information at all about those countries we are meant to lead. Worse, we have very little information about our own country and its past. That is why it is not really possible to compare a writer like Howells with any living American writer because Howells thought that it was a good thing to know as much as possible about his own country as well as other countries while our writers today, in common with the presidents and paint manufacturers, live in a present without past among signs whose meanings are uninterpretable.”

Gore Vidal

"William Dean Howells" (1983)
1980s, At Home (1988)

“First coffee, then a bowel movement. Then the Muse joins me.”

Gore Vidal

"Gore Vidal," interview by Gerald Clarke (1974), The Paris Review Interviews: Writers at Work, 5th series (1981)
1970s

“Professor Richard N. Current fusses, not irrelevantly, about the propriety of fictionalising actual political figures. I also fuss about this. But he has fallen prey to the scholar-squirrel's delusion that there is a final Truth revealed only to the tenured few in their footnote maze; in this he is simply naïve.”

Gore Vidal

"Lincoln and the Priests of Academe"
1990s, United States - Essays 1952-1992 (1992)

“I shared, naturally, in that hatred of organized labor which has been the one political constant in my lifetime, culminating in Ronald Reagan's most popular gesture, the smashing of the air-controllers' union. No alternative view of organized labor has ever come to us through the popular media. If labor leaders were not crooks like Jimmy Hoffa, they were in the pay of Moscow.”

Gore Vidal

Source: 1990s, Screening History (1992), Ch. 2: Fire Over England, p. 34