If you type “What’s my user agent” into Google, you’ll get something like this:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.79 Safari/537.36
Websites can use this to determine what browser and operating system you’re using, but most of the information is actually pretty pointless. In case you can’t tell, this is the user agent for Chrome.
Back in the day, when the Browser Wars were just heating up, Netscape pretty much owned the web. Many website owners decided to code their sites to specifically work with Netscape (which was once called Mozilla). To do this, they’d look at the browser’s user agent, which still said Mozilla 1.0 (or whatever version) rather than its commercial “Netscape” name.
When Microsoft wanted its Internet Explorer browser to compete with Netscape, they decided to spoof the user agent, adding “Mozilla” to the front of their own.
As other browsers emerged, they too spoofed previous browsers. KHTML was the browser engine for the Linux/Unix desktop environment KDE’s Konqueror browser. Apple borrowed its free source code to create Webkit, which was the engine for Safari. Google borrowed Apple’s source code in its own Chromium browser, which commercially became Chrome.
The Gecko part actually has nothing to do with Chrome, which is the actual browser featured in the example above. Gecko was the rendering engine for Netscape, Mozilla (the open source spin-off of Netscape) and eventually Mozilla Firefox (originally called Phoenix, then Firebird). Nevertheless, “like Gecko” still appears in many browsers’ user agents.