@lassoan, as far as I know, vtkProperty does not use sRGB color space by default unless you explicitly enable it using vtkRenderWindow::SetUseSRGBColorSpace.
However, when this option is enabled, the behavior you described appears to be a bug. Internally, the mapper converts the values to sRGB color space for PBR interpolation, regardless of the window setting. You can find the relevant code here: https://gitlab.kitware.com/vtk/vtk/-/blob/33e80863c0c3499993ce422b372d82f0546fc2b1/Rendering/OpenGL2/vtkOpenGLPolyDataMapper.cxx#L1459.
This behavior has been consistent since PBR was introduced in VTK, which you can review here: https://gitlab.kitware.com/vtk/vtk/-/merge_requests/5584/diffs#119ad58b35cad6a89360ea74ac1db4ca126c5816_900_1129.
To fix this, gamma correction should depend on whether the window utilizes sRGB color space.
@Michael FYI