  We characterize the magnetic ground state of the topological kagome metal
GdV$_6$Sn$_6$ via resonant X-ray diffraction. Previous magnetoentropic studies
of GdV$_6$Sn$_6$ suggested the presence of a modulated magnetic order distinct
from the ferromagnetism that is easily polarized by the application of a
magnetic field. Diffraction data near the Gd-$L_2$ edge directly resolve a
$c$-axis modulated spin structure order on the Gd sublattice with an
incommensurate wave vector that evolves upon cooling toward a partial lock-in
transition. While equal moment (spiral) and amplitude (sine) modulated spin
states can not be unambiguously discerned from the scattering data, the overall
phenomenology suggests an amplitude modulated state with moments predominantly
oriented in the $ab$-plane. Comparisons to the ``double-flat" spiral state
observed in Mn-based $R$Mn$_6$Sn$_6$ kagome compounds of the same structure
type are discussed.
