こう見えて魔術師なんだよ! ガラじゃないのは解ってるけど!
いや 魔術は見た目ではなく天性の物だから。。。
Upon failing another sword fight, the character says this and another character adds the second line in answer.
My reading of the first line is "Despite my appearances, I am a magician! It is obvious/clear it doesn't..."
To which another character adds "No, magic is not about one's appearance, but innate quality (such as genetic?)."
What is the exact nuance of ガラ? It seems like it could be "built" but also "not appropriate"?
Dictionaries I checked only say 柄 refers to one's innate quality, but I think there's more than that. This may seem tricky, but I feel ガラ (柄 in kanji) can refer to either one's (inner) personality or one's "persona" (shown externally).
Accordingly, 柄じゃない can mean both "not in line with my (true) personality" and "different from what others expect of me", depending on the context. The following two conversations seem both perfectly natural to me.
「メイド喫茶行かない？」
「ごめんなさい、私、そういう柄じゃないんです。」
"Why not go to a maid café?"
"Sorry, that's not really my style."
「メイド喫茶行かない？」
「えー、そういう柄じゃないでしょう？」
"Why not go to a maid café?"
"Hey, do I seem like someone who'd go there?"
In your case, I think 柄 refers to his "outward" personality or appearance. ガラじゃないのは解ってる means he knows he doesn't seem like a magician.
