My website serves gzipped content. I verified with Firebug and YSlow. However, JMeter does not request the gzipped content. Therefore, it gets all uncompressed content. As
Add an HTTP Header Manager to the Thread Group in your Test Plan.
Add the name-value pair:
Accept-Encodinggzip,deflate,sdchThis will ensure that all JMeter requests use HTTP compression.
To verify:
View the Results TreeSampler result tab for one of the webpages.Do you see these name-value pairs?
Content-Encoding: gzip
Vary: Accept-Encoding
Transfer-Encoding: chunked
If yes, then you've successfully setup gzip requests in JMeter. Congrats.
Another way to verify is in the Summary Report stats:
You'll see that the Avg Bytes values are the uncompressed sizes.  That's OK.  For whatever reason, that's how JMeter works. Pay attention to the KB/sec column.  That will show an improvement of 6-10x with gzip enabled.