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As rowan says, enabling gzip compression shouldn't have affected any of your users. If a browser doesn't support compression it will simply not request it from the server, in which case the site loads just as it did before.
Lower page-load times can boost a site's ranking in Google search results, so if you're concerned about SEO at all I'd leave compression enabled.
I bet what you're seeing is just a natural fluctuation in traffic.
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