We study the problem of gathering information from the nodes of a radio network into a central node. We model the network of possible transmissions by a graph and consider a binary model of interference in which two transmissions interfere if the distance in the graph from the sender of one transmission to the receiver of the other is $d_I$ or less. A {\em round} is a set of non-interfering transmissions. In this paper, we determine the exact number of rounds required to gather one piece of information from each node of a square two-dimensional grid into the central node. If $d_I = 2k-1$ is odd, then the number of rounds is $k(N-1)-c_k$ where $N$ is the number of nodes and $c_k$ is a constant that depends on $k$. If $d_I = 2k$ is even, then the number of rounds is $(k+\frac{1}{4})(N-1)-c'_k$ where $c'_k$ is a constant that depends on $k$. The even case uses a method based on linear programming duality to prove the lower bound, and sophisticated algorithms using the symmetry of the grid and non-shortest paths to establish the matching upper bound. We then generalize our results to hexagonal grids.