3ds Max is the historical pro-grade 3D authoring tool for GTA mapping. Tool overview here, not a tutorial.

What it does in the workflow

Builds 3D geometry. Materials and UV mapping. Skinning for animated objects. Exports via plugins to GTA-compatible formats (YDR/YDD).

Most professional MLO creators have 3ds Max in their pipeline.

Cost and licensing

Commercial. Significant license cost. Indie creators often work in Blender instead for cost reasons.

If you're putting together a creator setup from scratch, the lighter GTA 6 creator utilities (planners, generators, RP-economy calculators) are free and complement whatever 3D tool you settle on.