This paper studies a general form problem in which a lower bounded continuously differentiable function is minimized over a block separable set incorporating a group sparsity expression as a constraint or a penalty (or both) in the group sparsity setting. This class of problems is generally hard to solve, yet highly applicable in numerous practical settings. Particularly, we study the proximal mapping that includes group-sparsity terms, and derive an efficient method to compute it. Necessary optimality conditions for the problem are devised, and a hierarchy between stationary-based and coordinate-wised based conditions is established. Methods that obtain points satisfying the optimality conditions are presented, analyzed and tested in applications from the fields of investment and graph theory.