Gerber is a legacy brand, founded in 1939. Leatherman came along several decades later in 1983. Leatherman multitools are made in the USA and feature a 25 year warranty. On the other hand, Gerber multitools are made overseas but come with a lifetime warranty.
At the end of the day, both brands offer thoughtful design and high-quality construction. If you’re still unsure, compare similar models in each brand and decide which one you like more.