  Methods. We use the code MCFOST to solve the non-LTE problem of line
formation in non-axisymmetric accreting magnetospheres. We compute the
Br{\gamma} line profile originating from accretion columns for models with
different magnetic obliquities. We also derive monochromatic synthetic images
of the Br{\gamma} line emitting region across the line profile. This spectral
line is a prime diagnostics of magnetospheric accretion in young stars and is
accessible with the long baseline near-infrared interferometer GRAVITY
installed at the ESO Very Large Telescope Interferometer.
  Results. We derive Br{\gamma} line profiles as a function of rotational phase
and compute interferometric observables, visibilities and phases, from
synthetic images. The line profile shape is modulated along the rotational
cycle, exhibiting inverse P Cygni profiles at the time the accretion shock
faces the observer. The size of the line's emission region decreases as the
magnetic obliquity increases, which is reflected in a lower line flux. We apply
interferometric models to the synthetic visibilities in order to derive the
size of the line-emitting region. We find the derived interferometric size to
be more compact than the actual size of the magnetosphere, ranging from 50 to
90\% of the truncation radius. Additionally, we show that the rotation of the
non-axisymmetric magnetosphere is recovered from the rotational modulation of
the Br{\gamma}-to-continuum photo-centre shifts, as measured by the
differential phase of interferometric visibilities.
