Dependence on fertilizers limits the sustainability of tropical agriculture. Remineralization using rock byproducts offers a solution that is conditioned by the mineralogical–soil interaction. This study evaluated the agronomic and geochemical potential of a gabbro rock byproduct (GRB) as a bulk amendment in yellow maize (Zea mays L.) cultivation in Atlántico, Colombia. The specific objectives were (1) to characterize the mineralogy and geochemistry of the local GRB; (2) to quantify its neutralizing and fertilizing effect in an acidic Arenosol soil; and (3) to evaluate the biometric response of yellow maize (Zea mays L.) in a field trial. The trial was conducted in an acidic haplic Arenosol (pH 5.4) in 2023, with a 70-day cycle, comparing three management systems: M1 (control), M2 (47 Mg·ha⁻¹ GRB, seed type: ICA-109), and M3 (47 Mg·ha⁻¹ GRB, seed type: V-114). The assessed GRB, with 52.75% SiO₂ and 5.46% CaO, is rich in calcic plagioclase, clinopyroxenes, and zeolites. Application of GRB at 47 Mg·ha⁻¹ in treatment M3 coincided with marked changes in soil properties over the course of the trial, with pH rising from 5.4 to 6.4, cation exchange capacity from 5.0 to 12.1 cmol_c·kg⁻¹, and available phosphorus from 9.8 to 35.0 mg·kg⁻¹. Plants in M3 showed statistically significant increases (p < 0.001) in ear weight (median: 150 g vs. 60.5 g in M1) and in vegetative development. Because the trial was pseudo-replicated, used a high single-dose “shock-loading” rate, involved different maize genotypes across treatments, and covered only one 70-day cycle, these results should be interpreted as exploratory and site-specific. Even so, they indicate that GRB can act as an effective acidity corrector and slow-release multinutrient source under Arenosol conditions, with relevance for circular-economy strategies. Future work should evaluate agronomically doses, include replicated multi-cycle trials, and incorporate comparative and risk-assessment analyses.