gzipping assets is one of the simplest and most effective things you can do to make your web site faster.
For text files we are sending over networks, gzip is a big win and a no-brainer. However, images and other binary files should already be aggressively compressed, and you usually won’t benefit from gzipping them. In fact, binary files can increase a tiny bit in size because of added gzip headers. See this post for more details.