GZIP/DEFLATE push content compression is not supported for some reasons:

When in streaming mode, it does not seem to be safe enough to compress individual chunks while maintaining broad compatibility with clients and intermediaries.
In any case, Lightstreamer pushed content is already compressed because only the deltas in the item state is pushed. So, applying a further GZIP/DEFLATE-style compression is many cases could not provide any benefit (unless large strings are pushed, but this is not a common use case).

When in polling mode, most of the response length in many cases is given by the HTTP header, which would not change with GZIP/DEFLATE compression.

But we keep improving our protocol, so I don't exclude we could decide to provide some form of compression support in the future.