Blender is the free 3D tool. With community plugins it handles most of what 3ds Max does for GTA mapping.

Where Blender works

Geometry creation. Materials. UV mapping. Skinning. Community plugins handle export to GTA formats.

The plugin support is community-maintained and varies in quality.

Where Blender lags

Some advanced rigging features. Some specialist export workflows. Most pro MLO creators still use 3ds Max for these reasons.

The gap has been narrowing each year.