GZip + Smile/JSON gives a better size reduction over Smile (uncompressed) for sure. I myself use GZip + JSON mostly. The purpose of this article was to introduce Smile which is highly efficient when compared to JSON or BSON.

I found an interesting result for the same over internet for a dataset of size 133MB:
JSON(Uncompressed) 133MB
JSON(gzip) 16MB
Smile(Uncompressed) 72MB
Smile(gzip)16MB
BSON(Uncompressed)129MB
BSON(gzip)19MB

Reference: https://www.lucidchart.com/techblog/2019/12/06/json-compression-alternative-binary-formats-and-compression-methods/
Results: https://docs.google.com/spreadsheets/d/1FtyFsMf9aQVoFvib7ByWmW1KiDIAFcAoRQX4NYiagJc/edit#gid=427836680