Please NEVER use Round up anything EVER!!!!!!! This is a very dangerous and deadly product line. No professionals with any environmental knowledge use it here in Western NC.
Are you talking about wild garlic? Domestic garlic? Is it in your lawn too? Dig & pull especially aftre a nice rain, put something like a small plastic flower pot or piece of plastic on top to smother, or if you're sure it's garlic and not a "death camma" eat te tops in salad. They come from a tiny bulb which can be eaten too. Also don't allow them to bloom/seed. Many plantts can be killed with hot vinegar.
Bottom line is never use Monsanto anything. There is a huge environmental problem and damage to heirloom seed with tis vie stuff. They sue traditional farmers into extinction because the monsanto plants cross with neighbors and the next gernerations of the farmer's family seed then contains Monsanto DNA. Monsnto has the money to win, the farmers do not. We must keep a variety of seeds out there.