Looking for a quote to back up the question: what does Pip learn about being a gentleman?

Pip has tried so very hard to be a gentleman. With the assistance of Abel Magwitch's bequest, he has been able to fulfil a lifelong dream and start acting out the role of man about town. He throws himself enthusiastically into a world of fine clothes, parties, gambling, and all-round gaiety. But it's all so incredibly shallow. Pip has come to think that being a gentleman is all about what's on the outside. When Joe comes to London to pay him a visit, Pip is incredibly embarrassed by his awkward manners and simple country demeanor; poor Joe is truly a fish out of water in the big city. When Pip sees Joe, he's ashamed to discover about himself that he's turned into something of a snob.
Later on in the story, when Pip's recovering from illness, he awakes to see Joe's kindly face by his bedside. Pip is immediately overcome by remorse for the way he treated Joe earlier on his visit to London:

O Joe, you break my heart! Look angry at me, Joe. Strike me, Joe. Tell me of my ingratitude. Don’t be so good to me!’ For, Joe had actually laid his head down on the pillow at my side and put his arm round my neck, in his joy that I knew him. ‘Which dear old Pip, old chap,’ said Joe, ‘you and me was ever friends. And when you’re well enough to go out for a ride - what larks!

Joe's kindness, compassion and unbreakable friendship has shown Pip what it really means to be a gentleman. Money, clothes, an active social life––none of that means anything. What really matters is what's inside and how you treat other people.