I'd like to know the meaning of the phrase "he was great for that" in the following excerpt from the last chapter of The Great Gatsby by F. Scott Fitzgerald:
"Jimmy was bound to get ahead. He always had some resolves like this or something. Do you notice what he's got about improving his mind? He was always great for that. He told me I et like a hog once and I beat him for it."
Does it mean that "he took a great interest in that"? "he was keen on it" or that "he was good at that", because usually achieved what he set out to do?
You are on the right track. In fact, you are exactly right. The meaning of the phrase works as a combination of the suggestion of talent and interest and dedication.
The phrase, "great for that", here is meant to express the idea that he had a great interest in self-improvement and a dedication to it. "Great" relates to extremity or an idea of quantifying Gatsby's feelings toward the idea improving his mind. "For" refers to the nature of Gatsby's feelings, as he is "pro", positive, or supportive of the idea of improving his mind.
We might replace the phrase, "great for that", with "greatly involved in that", "thoroughly approving of that", "deeply involved in that", or "passionately dedicated to that".
Gatsby was great for improving his mind like a professional athlete would be great for improving his/her body, or like a singer is great for music.
In saying that Gatsby was "great for" improving his mind, Gatsby's father is stating that Gatsby was a driven individual, focused on setting and reaching goals. Nothing would get in his way - not family and not personal weakness.
Nick also recognizes Gatsby's characteristic self-determination in this line:
“The truth was that Gatsby of West Egg, Long Island, sprang from his Platonic conception of himself.”