From 2016 to 2019, Will Smith was one of the better relievers in all of baseball during his time with the Giants and found himself being one of the more desirable relievers on the market despite the fact that the Giants offered him a qualifying offer. The Braves ended up paying up to pry Smith away from San Francisco and he ended up winning a World Series with them as their closer in 2021. The last few years have been tough for Smith as he hasn't been nearly as effective since leaving San Francisco, but he is headed to the World Series with the Rangers where he posted a 4.40 ERA in 60 appearances with them in 2023
Chris Stratton has turned himself into a fine MLB reliever, that it sure didn't seem like that was the direction his career was headed during his time with the Giants. San Francisco's first round pick in the 2012 MLB Draft was originally a starting pitching prospect and a good one, but starting didn't work out well for him with the Giants. San Francisco ended up trading him to the Angels and later bounced around the league, converted to being a bullpen arm with Pittsburgh, and eventually landed with the Rangers at the trade deadline this year where he has become one of Texas' more reliable relievers.
This one is cheating a little bit, but this sort of list would feel wrong if it didn't include the former Giants skipper. After a long stint with the Padres as their manager, Bochy came to San Francisco in 2007 and proceeded to win three World Series titles with the Giants and has become a bit of a managerial legend. He retired after the 2019 season, but the Rangers convinced him to come out of retirement for this season and, well, that decision has seemingly worked out well for Texas.