The two showed up at Patriots Fantasy Camp -- in which team members put paying fans through the paces of a regular day in the life of a Pats player. The money raised goes to charity.

Nobody really knew if Brady and Gronk would show up -- since they've been absent from New England's voluntary workouts the past couple weeks ... leading Bill Belichick to throw some not-so-subtle shade on his Hall of Fame QB.

But, when the event began Monday morning, the two most famous players were there.

Both players are expected to return for mandatory minicamp on Tuesday ... and their attendance at Monday's annual Patriots Fantasy Camp is a good sign that'll actually happen.