There's somewhere around $500B left of stock buybacks waiting to process after earnings in Q4. I think there's a good chance market will end the year higher than it is now although, there are lots of factors trade war and geopolitical risk factors that can prevent that from happening.
Bottom line, if it was so obvious that the market would be sharply lower by the end of the year than it is now, then that would have been priced in already. Long term, I agree with Gundlach.