  We report the first measurement of the inclusive jet and the dijet
longitudinal double-spin asymmetries, $A_{LL}$, at midrapidity in polarized
$pp$ collisions at a center-of-mass energy $\sqrt{s} = 510$ GeV. The inclusive
jet $A_{LL}$ measurement is sensitive to the gluon helicity distribution down
to a gluon momentum fraction of $x\approx 0.015$, while the dijet measurements,
separated into four jet-pair topologies, provide constraints on the $x$
dependence of the gluon polarization. Both results are consistent with previous
measurements made at $\sqrt{s}= 200$ GeV in the overlapping kinematic region,
$x > 0.05$, and show good agreement with predictions from recent
next-to-leading order global analyses.
