Something I just thought of, which I feel people are missing while making the Ace comparisons is that Dragon and Ace are completely different types of prisoners.
Dragon did a mutiny, pretty bad but Garp could still bail him out. He also probably wasn't sent to Impel Down and he wasn't super well known.
Ace was a well known pirate, the Pirate King's son during tge greatest era of piracy, and the protege of the World's Strongest Man. He was very influential and symbolic, and his death was something the WG wanted to use to crush the spirits of pirates.
Also, Garp knew that WB would come for Ace. It was a similar situation to Kizaru on EH where he knew someone else woild pick up his slack.
With Dragon I'm guessing he was the only one who could've helped at the time.