What does Gabelle tell the future Everemonde heir to do?
I assume you are referring to Gabelle's letter to the future Marquis of Everemonde, informing him of his dangerous position and begging for help to be released as he has been imprisoned by the forces of the Revolution for following the orders that he was given and faithfulness to his masters.
This letter comes in Chapter 24, and has been delivered to Tellson's Bank where many of the French aristocracy who have fled France have been drawn. Darnay tells Mr. Lorry that he will ensure the letter is delivered to the Evremonde heir, never once revealing that he is the same individual. The letter contains a heartfelt plea:
For the love of Heaven, of justice, of generosity, of the honour your noble name, I supplicate you, Monsieur heretofore the Marquis, to succour and release me. My fault is, that I have been true to you. Oh Monsieur heretofore the Marquis, I pray you be you true to me!
In such terms Darnay cannot but refuse to feel a sense of responsibility for his servant who is only asking for the same fidelity that he has shown his master himself.