Blender is the free 3D application. With community plugins it can do most of what 3ds Max can for GTA mapping.

Where Blender works

Geometry creation. Materials. UV mapping. Skinning. Exporting to GTA file formats is done by community plugins.

Plugin support is community-maintained, and quality varies.

Where Blender lags

Some advanced rigging capabilities. Some specialist export pipelines. These are the reasons why most professional MLO makers still use 3ds Max.

The gap gets smaller every year.