We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time that g-mode seismology can be exploited quantitatively for stars on the extreme horizontal branch, all previous successful seismic analyses having been confined so far to short-period, p-mode pulsators. We demonstrate that current models of hot B subdwarfs can quite well explain the observed g-mode periods, while being consistent with independent constraints provided by spectroscopy. We identify the 18 pulsations retained in our analysis as low-degree ({l}=1 and 2), intermediate-order (k=-9 through -58) g-modes. The periods (frequencies) are recovered, on average, at the 0.22% level, which is comparable to the best results obtained for p-mode pulsators. We infer the following structural and core parameters for KPD 1943+4058 (formal fitting uncertainties only): T_eff_=28050+/-470K, logg=5.52+/-0.03, M_*_=0.496+/-0.002M_{sun}_, log(M_env_/M*)=-2.55+/-0.07, log(1-M_core_/M_*_)=-0.3+/-0.01, and X_core_(C+O)=0.261+/-0.008. We additionally derive the age of the star since the zero-age extended horizontal branch 18.4+/-1.0Myr, the radius R=0.203+/-0.007R_{sun}_, the luminosity L=22.9+/-3.13L_{sun}_, the absolute magnitude M_V_=4.21+/-0.11, the reddening index E(B-V)=0.094+/-0.017, and the distance d=1180+/-95pc.