Cu and Zn are said to compete with Gd for a place on GBCA chelation ligands, yet clearly so does iron. Gd exchange with transferrin seems at the core of observations of interference with iron and calcium assays seem with weakly chelating agents and iron loaded patients regularly exhibit high transferrin saturation ratios. 

Fretellier N, Poteau N, Factor C, et al. Analytical interference in serum iron determination reveals iron versus gadolinium transmetallation with linear gadolinium-based contrast agents. Investigative Radiology 2014;49(12):766-772.

Vorobiov M, Basok A, Tovbin D, Shnaider A, Katchko L, Rogachev B. Iron-mobilizing properties of the gadolinium-DTPA complex: clinical and experimental observations. Nephrology Dialysis Transplantation 2003;18(5):884-887.