The present study proposes a Gram-matrix-based sensor placement strategy for sparse force reconstruction in the frequency domain. A modal decomposition of the Gram matrix reveals that its structure is dominated by a few modes near the target frequency, and that each modal contribution reflects the spatial correlation of the corresponding mode shape. This suggests that placing sensors near nodal regions where spatial correlation is low can reduce coherence in the frequency response function (FRF) matrix and improve force reconstruction accuracy. To translate the physical insight into a practical design framework, a greedy algorithm is proposed to select sensor locations that minimize the off-diagonal energy of the Gram matrix. Numerical simulations and experimental validations demonstrate that the proposed method yields robust and accurate force estimation, outperforming heuristic sensor layouts.