Optaplanner is a heuristic approach, meaning that you will not have any optimality guarantees until you have explored/filtered all of the domain. Gurobi is an MILP solver, relying on linear relaxations of the integer problem to provide you an optimality gap as you keep searching the tree and eventually a proof of optimality once you found one of the optimal solutions (there might be more than one that achieve the same objective, specially in scheduling)
Me: "Why is it downloading an .exe?"