  We calculated the dimensionless gyromagnetic ratio ("$g$-factor") of
self-gravitating, uniformly rotating disks of dust with a constant specific
charge $\epsilon$. These disk solutions to the Einstein-Maxwell equations
depend on $\epsilon$ and a "relativity parameter" $\gamma$ ($0<\gamma\le 1$) up
to a scaling parameter. Accordingly, the $g$-factor is a function
$g=g(\gamma,\epsilon)$. The Newtonian limit is characterized by $\gamma \ll 1$,
whereas $\gamma\to 1$ leads to a black-hole limit. The $g$-factor, for all
$\epsilon$, approaches the values $g=1$ as $\gamma\to 0$ and $g=2$ as
$\gamma\to 1$.
