On 22 January 2016, the Jewish Daily Forward published an editorial titled "Donald Trump Retweets Creator of Pic Showing Bernie Sanders Sent to Gas Chamber"; the title of the item was edited at an indeterminate point after it was published.
Confusion about the claim (evidenced in the examples reproduced above) may have resulted from what was a clumsy initial headline, the original preserved in early social media shares of the page:
As the embedded links initially demonstrated, the piece was originally titled "Donald Trump Retweets Pic Showing Him Sending Bernie Sanders to Gas Chamber." What Trump had done was to tweet a completely different image (featuring GOP rival Jeb Bush) created by the same account responsible for the "gas chamber" image, but he hadn't directly retweeted the Holocaust-inspired graphic:
Trump’s official Twitter feed retweeted this Tweet that takes a jab at GOP rival Jeb Bush. It shows a photoshopped image of Bush in front of the Trump Towers, holding up a "Vote for Trump" sign.
Seems innocuous enough. Until you see the account it's retweeted from: @WhiteGenocideTM.
But that's not even the worst of it. A @WhiteGenocideTM photoshop creation shows Trump in a Nazi uniform flipping on the gas switch on a gas chamber housing… Jewish candidate Bernie Sanders.
The editorial asserted that Trump's ostensible re-tweeting of material from an account populated largely by racist and anti-Semitic material was perhaps troubling. However, the chain of responsibility for the re-tweet remained a matter of dispute, as the Trump tweet in question occurred nine days after the offensive "gas chamber" image was reportedly first published by @WhiteGenocideTM. Whether Trump was even aware such an image previously appeared on @WhiteGenocideTM's timeline was unclear and perhaps improbable.
We attempted to validate the existence of the original 13 January 2016 "gas chamber" tweet on @WhiteGenocideTM's Twitter feed, but if it had once been published there, it was deleted at some point prior 24 January 2016. Jewish Daily Forward shared a screenshot of the purported tweet, however:
A significant amount of scrolling was required to ascertain that the reported tweet from 13 January 2016 was no longer available. However, the account in question did primarily publish material of a similar bent, and Trump's 22 January 2016 re-tweet remained intact:
— Donald J. Trump (@realDonaldTrump) January 22, 2016
As the Jewish Daily Forward piece stated, Donald Trump re-tweeted an unrelated, photoshopped image (of Jeb Bush holding a "Vote Trump" sign outside Trump Tower in New York City) on 22 January 2016. The article's original headline suggested that Trump re-tweeted an image of himself placing rival Bernie Sanders in a gas chamber, but it was later changed (without an editor's note explaining the significant modification). Due to the nature and timing of re-tweets, it's likely Trump was unaware of the nature of the account from which the image originated.