gr() is deprecated as a public formula marker in drmTMB 0.1.3.9000.
Use relmat() for a validated lower-level user-supplied relatedness or
precision matrix, animal() for pedigree or additive-relatedness animal
models, phylo() for phylogenetic dependence, or spatial() for spatial
dependence. The exported gr() placeholder remains only for compatibility
with older design notes and should not be used in new model formulas.