  We derive new constraints on the $E_G$ statistic as a test of gravity,
combining the CMB lensing map estimated from Data Release 6 (DR6) of the
Atacama Cosmology Telescope with SDSS BOSS CMASS and LOWZ galaxy data. We
develop an analysis pipeline to measure the cross-correlation between CMB
lensing maps and galaxy data, following a blinding policy and testing the
approach through null and consistency checks. By testing the equivalence of the
spatial and temporal gravitational potentials, the $E_G$ statistic can
distinguish $\Lambda$CDM from alternative models of gravity. We find $E_G=
0.31^{+0.06}_{-0.05}$ for ACT and CMASS data at 68.28\% confidence level, and
$E_G = 0.49^{+0.14}_{-0.11}$ for ACT and LOWZ. Systematic errors are estimated
to be 3\% and 4\% respectively. Including CMB lensing information from Planck
PR4 results in $E_G = 0.34^{+0.05}_{-0.05}$ with CMASS and $E_G=
0.43^{+0.11}_{-0.09}$ with LOWZ. These are consistent with predictions for the
$\Lambda$CDM model that best fits the Planck CMB anisotropy and SDSS BOSS BAO,
where $E_G^{\rm GR} (z_{\rm eff} = 0.555) = 0.401\pm 0.005$ for CMB lensing
combined with CMASS and $E_G^{\rm GR} (z_{\rm eff} = 0.316) = 0.452\pm0.005$
combined with LOWZ. We also find $E_G$ to be scale independent, with PTE
$>5\%$, as predicted by general relativity. The methods developed in this work
are also applicable to improved future analyses with upcoming spectroscopic
galaxy samples and CMB lensing measurements.
