There isn't a lot of information available, but Mat Gamel started the full team workout doing running drills and didn't make it further than that. He apparently spoke with assistant athletic trainer Dave Yaeger, rode a cart back to the facilities and didn't return.
The Brewers say he aggravated the knee and they are being extra cautious with his return from the ACL tear and surgery.
Adam McCalvy wrote about Gamel and his return this week, focusing on Gamel's use of a heavy brace on the knee.
Mat Gamel Leaves First Spring Workout After Aggravating Knee Injury
Brew Crew Ball | Feb 17