[chris vickery]

Jordan, if you want to kill the gloss in your paint, use regular laquer thinner as your reducer. Gasoline may work fine but I'd rather use an actual paint component than use gas. Another method to dull semi or gloss paint is to overcoat the first coat just after it has flashed off - the undercoat will pull the thinners out of the second coat resulting in a flat appearance, although sometimes it will end up blotchy.
Failing these solutions, go back to the paint place and ask for a flatting agent to add to your batch. The only problem with this is that flatting agent is usually quite pricey and often you need two quarts to a gallon to achieve the proper effect. Flatting agents usually around $30 to $50 a quart!