Who was Gabelle?

Théophile Gabelle, to give him his full name, is the man charged with taking care of the Evrémonde estate after the Marquis's death. Gabelle has been acting as Charles Darnay's steward, which immediately makes him suspicious in the eyes of the revolutionary authorities. At this tumultuous period in French history, simply associating with an aristocratic family, let alone working for one, is enough to mark you down as a traitor to the nation. That being the case, it comes as no surprise when Gabelle's slung in jail by the revolutionaries.
From his stinking prison cell, he sends an SOS to Darnay, informing him of his plight. Darnay, safely ensconced in England, could choose to ignore Gabelle's letter. But Darnay wouldn't be Darnay if he did that, for this is a man with a profound sense of duty, a unswerving devotion to doing the right thing. Feeling more than a little guilty over Gabelle's imprisonment—Gabelle was, after all, slung in jail for serving Darnay—Charles determines to go back to Revolutionary France and settle all unfinished business.
It's a brave move and a very dangerous one, because if Darnay's identity is discovered by the revolutionaries, he could well find himself sharing the same prison cell as Gabelle. Even worse, he could end up losing his head to the guillotine.