  The X-ray afterglow plateau emission observed in many Gamma-ray Bursts (GRBs)
has been interpreted as either being fueled by fallback onto a newly formed
black hole, or by the spin-down luminosity of an ultra-magnetized millisecond
neutron star. If the latter model is assumed, GRB X-ray afterglow light curves
can be analytically reproduced. We fit a sample of GRB X-ray plateaus,
interestingly yielding a distribution in the magnetic field versus spin period
(B-P) diagram consistent with $B\propto P^{7/6}$. This is expected from the
well-established physics of the spin-up line minimum period for Galactic
millisecond pulsars. The normalisation of the relation we obtain perfectly
matches spin-up line predictions for the expected masses ($\sim 1 M_{\odot}$)
and radii ($\sim 10 {\rm ~km}$) of newly born magnetars, and mass accretion
rates consistent with GRB expectations of $10^{-4} M_{\odot}/{\rm s} <\dot{M}<
10^{-1} M_{\odot}/{\rm s}$. Short GRBs with extended emission (SEE) appear
towards the high period end of the distribution, while the long GRBs (LGRBs)
towards the short period end. This result is consistent with spin-up limit
expectations where the total accreted mass determines the position of the
neutron star in the B-P diagram. The P-B distribution for LGRBs and SEE are
statistically different, further supporting the idea that the fundamental plane
relation \citep{dainotti16c,Dainotti2017} is a powerful discriminant among
those populations. Our conclusions are robust against suppositions regarding
the GRB collimation angle and magnetar breaking index, which shifts the
resulting magnetar properties parallel to the spin-up line, and strongly
support a magnetar origin for GRBs presenting X-ray plateaus.
