Sand-rubber mixtures (SRMs) and gravel-rubber mixtures (GRMs) can be designed to have outstanding engineering characteristics. However, in comparison with the extensive investigations carried out on SRMs, comprehensive studies characterizing GRMs are very limited, making the index, physical, and mechanical properties of such synthetic materials mostly unknown. To facilitate the use of GRMs as fill materials in geotechnical applications, this paper provides a summary of the engineering properties of GRMs obtained from detailed laboratory tests (macro-scale response) and discrete element method (DEM) numerical investigations (micro-scale response) conducted at the University of Canterbury, New Zealand, over the past five years. The experimental investigations indicate that GRMs have adequate strength and compressibility properties for use as structural fills. Moreover, GRMs can be utilized also as filters for dampening vibrations and seismic waves, as the addition of rubber aggregates enhanced their energy absorption and dynamic properties. The DEM numerical study provides important insights into the fabric and force anisotropy, load-transfer mechanism, and strong-force network at the particle-to-particle scale, confirming the existence of three distinct behavioral zones for GRMs: rigid gravel-like, intermediate/dual, and soft rubber-like. Dual-behavior GRMs have high ductility that prevents abrupt failure and allows the mixtures to withstand larger plastic deformation before failure; they are suitable energy-absorbing structural fills for foundations and embankments as well as backfill materials for retaining structures.