In this sense "force" means "interaction". Gravity is a kind of fundamental interaction, with its own coupling constant -- Newton's constant. Yes, gravity is treated as curvature of space-time. This curvature is dynamical, and every particle that exists in space-time should feel the curvature, and in turn create its own. In other words particles couple to the curvature and couple to each other via the curvature. That's why gravity is interaction.
Unification of all the fundamental interactions means describing them using one theoretical framework. Why? Because they are all part of the same reality. Sure we can mostly ignore gravity at present due to its weakness. But gravity becomes strong in extreme conditions. The two famous examples are viscinity of a black hole, and the very early universe. In those cases gravity can become comparable in strength with the other three interactions. So, the full understanding of those extremes requires a unified framework for General Relativity (a classical field theory that describes gravity) and Quantum Field Theory (framework for particle physics). This unified framework is propably what you mean by "superforce". To my knowledge, currently the only consistent such framework (that would automatically include all four interactions) is string theory. But it is still work in progress.
Just a as a side note, your "superforce" and Grand Unification are different. Grand Unification doesn't include gravity, hence doesn't require the unified framework I described above.