Filler-induced strengthening is ubiquitous in material science and is
particularly well-established in polymeric nanocomposites. Despite having
similar constituents, colloidal gels with solid filling exhibit distinct
rheology, which is poorly understood. We show using experiments and simulations
that filling monotonically enhances the yield stress of colloidal gels, while
the elastic modulus first increases before decreasing. The latter effect
results from a disturbed gel matrix at dense filling, evidenced by a growing
inter-phase stress. This structural frustration is, however, not detrimental to
yielding resistance. Instead, fillers offer additional mechanical support to
the gel backbone via percolating force chains, at the same time decreasing the
yield strain. We develop a mechanistic picture of this phenomenology that leads
us to a novel `filler-removal protocol,' making possible individual control
over the strength and brittleness of a composite gel