I am working since a few months with gurobi and did figure out, that the gap says how far a feasible solutions is away from the optimal solution. However, what does say it exactly?
The gurobi docs says it depends on the upper and lower objective bound. I figured out, that this depends to duality.
As I did understand this topic: The lower bound descripes an objective value for the lowest values of the decision variables that is still feasible. The upper bound does descripe the highest values for feasible objective value. So the gap for a feasible solution descripes the distances between those bounds. However, which one do the solver exactly know and which one is a solution the solver founds during the branch-and-bound process?