Surely the impact of Webkit has been greater than that of Gecko? Webkit made decent mobile browsing possible, and until recently Gecko wasn't available in mobile browsers.
Gecko did put a dent in desktop browser usage share early on (when Safari was Mac-only, and Safari for Windows has never caught on – for good reasons), but the desktop is quickly becoming the second screen. Nowadays, most sites are built for Webkit first.
Safari was released in January of 2003. Soon after, Microsoft ceased developing Internet Explorer for Mac. Almost two years later, Firefox 1.0 was released, and it took quite some time after that for it to become a good browser. In the meanwhile, plenty of us had been using a Gecko based browser for years: Netscape.
I'd make the argument that without Gecko the web would have become even more entangled with and dependent on IE. That would have smothered WebKit in the cradle, making it impossible for anyone outside of Microsoft to make a decent mobile web browser.
Gecko did put a dent in desktop browser usage share early on (when Safari was Mac-only, and Safari for Windows has never caught on – for good reasons), but the desktop is quickly becoming the second screen. Nowadays, most sites are built for Webkit first.