The gift of oysters is given out of total and complete guilt, because Pip made a visit back home, and he didn't go to say hello or to visit Joe. He is going back to the village to see Estella and talk with her and Miss Havisham--he assumes that Miss Havisham is his benefactress, and that Estella has been groomed for him and vice-versa, so he wants to check in with them. But that means going to the village. And, as he does, he dreads having to stay with Joe, because Joe had just visited him in London, and it was a horribly awkward affair that had gone really poorly. Joe had behaved strangely, and it had not been a good experience. So, Pip didn't want to have to visit Joe and go through that awkwardness again. To make up for his lack of a visit, he
"sent a penitential codfish and barrel of oysters to Jow (as reparation for not having gone myself)."
Penitential refers to the word penitent, which means repentant or regretful, and reparation means an attempt to make it better. So, it is an apology gift, a "I'm sorry I'm not visiting...will the oysters make you less offended?" kind of gift.
This gift is not appropriate, because it is offensive and rude. Imagining visiting the very neighborhood of your parents after being gone for a while, but not even dropping by to say hello. Pip has been very slack in his relationship with Joe since he got his money, and has almost cut off all communication. He doesn't write often, is embarrassed by their station, and treats them poorly. It is very rude to not have visited. Oysters mean nothing to the lack of a visit, and might even be more offensive than not doing anything would have been--it's an admission of their unworthiness of his presence. I hope that those thoughts helped; good luck!