  We study the Callan-Gross ratio $R={\rm d}\sigma_L/{\rm d}\sigma_T$ in
heavy-quark pair leptoproduction, $lN\rightarrow l^{\prime}Q\bar{Q}X$, as a
probe of linearly polarized gluons inside unpolarized proton, where ${\rm
d}\sigma_T$ (${\rm d}\sigma_L$) is the differential cross section of the
$\gamma^*N\rightarrow Q\bar{Q}X$ process initiated by a transverse
(longitudinal) virtual photon. Note first that the maximal value for the
quantity $R$ allowed by the photon-gluon fusion with unpolarized gluons is
large, about 2. We calculate the contribution of the transverse-momentum
dependent gluonic counterpart of the Boer-Mulders function, $h_{1}^{\perp g}$,
describing the linear polarization of gluons inside unpolarized proton. Our
analysis shows that the maximum value of the ratio $R$ depends strongly on the
gluon polarization; it varies from 0 to $\frac{Q^2}{4m^2}$ depending on
$h_{1}^{\perp g}$. We conclude that the Callan-Gross ratio in heavy-quark pair
leptoproduction is predicted to be large and very sensitive to the contribution
of linearly polarized gluons. For this reason, future measurements of the
longitudinal and transverse components of the charm and bottom production cross
sections at the proposed EIC and LHeC colliders seem to be very promising for
determination of the linear polarization of gluons inside unpolarized proton.
