If you are serving a .gz file, perhaps you are looking to add an association between the file extension and the desired mime-type with mod_mime. The following can be added to the VirtualHost configuration.
AddEncoding x-gzip .gz
Alternatively, a more complete example for serving pre-compressed content involves not just adding the encoding-type but also disabling mod_deflate and resetting the content-type. This example uses mod_header to add the encoding-type.
<IfModule mod_headers.c>
# Serve gzip compressed CSS files if they exist
# and the client accepts gzip.
RewriteCond "%{HTTP:Accept-encoding}" "gzip"
RewriteCond "%{DOCUMENT_ROOT}%{REQUEST_FILENAME}\.gz" -s
RewriteRule "^(.*)\.css" "$1\.css\.gz" [QSA]
# Serve gzip compressed JS files if they exist
# and the client accepts gzip.
RewriteCond "%{HTTP:Accept-encoding}" "gzip"
RewriteCond "%{DOCUMENT_ROOT}%{REQUEST_FILENAME}\.gz" -s
RewriteRule "^(.*)\.js" "$1\.js\.gz" [QSA]
# Serve correct content types, and prevent mod_deflate double gzip.
RewriteRule "\.css\.gz$" "-" [T=text/css,E=no-gzip:1]
RewriteRule "\.js\.gz$" "-" [T=text/javascript,E=no-gzip:1]
<FilesMatch "(\.js\.gz|\.css\.gz)$">
# Serve correct encoding type.
Header append Content-Encoding gzip
# Force proxies to cache gzipped &
# non-gzipped css/js files separately.
Header append Vary Accept-Encoding
</FilesMatch>
</IfModule>
(Note the above adds DOCUMENT_ROOT to the RewriteCond file path from the referenced sample since I believe it is needed when used in the VirtualHost context from 2.2+.)