Drew Gilbert fell out of favor with the New York Mets the last two seasons. Despite a hot couple of weeks in Triple-A, the team didn’t view him as a potential center field solution this year. So, they traded him to the San Francisco Giants in their package to acquire Tyler Rogers.
A part of the reason why Gilbert became expendable is, according to SNY regular John Harper, due to their love for another.
Should also note: heard that while Mets had fallen out of love with Gilbert, they're very much in love with Carson Benge, convinced he's on a fast track to be an impact hitter.