This'd be worth testing. Isn't the way gzip works to build up a dictionary of characters/phrases and then store pointers to the dictionary? I'm not sure you can compress { or } but }); is another story.
ASCII is extremely repetitive. 1MB of random braces gzips down to 163KB. That's a compression ratio of 6.1:1. Optimal compression would be close to 8:1 (since each byte really contains only one bit of information).
Your sample data is bunk for determining the compression characteristics of braces in a realistic json file.
Huffman coding works by replacing common symbols with shorter codes and uncommon symbols with longer codes.
The codes used are therefore variable-length.
The codes are created in such a way that no code is the prefix of any other code.
That way, the decoder is able to know when it has reached the end of a code without the need for extra information other than the Huffman tree, which tells the decoder which codes belong to which symbols.
The Huffman tree can be pre-agreed or, more commonly, included with the compressed data.