  Evidence from different probes of the stellar initial mass function (IMF) of
massive early-type galaxies (ETGs) has repeatedly converged on IMFs more
bottom-heavy than in the Milky Way (MW). This consensus has come under scrutiny
due to often contradictory results from different methods on the level of
individual galaxies. In particular, a number of strong lensing probes are
ostensibly incompatible with a non-MW IMF. Radial gradients of the IMF --
related to gradients of the stellar mass-to-light ratio $\Upsilon$ -- can
potentially resolve this issue. We construct Schwarzschild models allowing for
$\Upsilon$-gradients in seven massive ETGs with MUSE and SINFONI observations.
We find dynamical evidence that $\Upsilon$ increases towards the center for all
ETGs. The gradients are confined to sub-kpc scales. Our results suggest that
constant-$\Upsilon$ models may overestimate the stellar mass of galaxies by up
to a factor 1.5. For all except one galaxy, we find a radius where the total
dynamical mass has a minimum. This minimum places the strongest constraints on
the IMF outside the center and appears at roughly 1 kpc. We consider the IMF at
this radius characteristic for the main body of each ETG. In terms of the IMF
mass-normalization $\alpha$ relative to a Kroupa IMF, we find on average a
MW-like IMF $<\alpha_{main}> = 1.03 \pm 0.19$. In the centers, we find
concentrated regions with increased mass normalizations that are less extreme
than previous studies suggested, but still point to a Salpeter-like IMF,
$<\alpha_{cen}> = 1.54 \pm 0.15$
