  We revisit the Tidally Excited Oscillations (TEOs) in the A-type,
main-sequence, eccentric binary KOI-54, the prototype of heartbeat stars.
Although the linear tidal response of the star is a series of orbital-harmonic
frequencies which are not stellar eigenfrequencies, we show that the
non-linearly excited non-orbital-harmonic TEOs are eigenmodes. By carefully
choosing the modes which satisfy the mode-coupling selection rules, a period
spacing ($\Delta P$) pattern of quadrupole gravity modes ($\Delta P \approx
2520-2535$ sec) can be discerned in the Fourier spectrum, with a detection
significance level of $99.9\%$. The inferred period spacing value agrees
remarkably well with the theoretical $l=2,m=0$ g modes from a stellar model
with the measured mass, radius and effective temperature. We also find that the
two largest-amplitude TEOs at $N=90, 91$ harmonics are very close to resonance
with $l=2,m=0$ eigenmodes, and likely come from different stars.
  Previous works on tidal oscillations primarily focus on the modeling of TEO
amplitudes and phases, the high sensitivity of TEO amplitude to the frequency
detuning (tidal forcing frequency minus the closest stellar eigenfrequency)
requires extremely dense grids of stellar models and prevents us from
constraining the stellar physical parameters easily. This work, however, opens
the window of real tidal asteroseismology by using the eigenfrequencies of the
star inferred from the non-linear TEOs and possibly very-close-to-resonance
linear TEOs. Our seismic modeling of these identified eigen g-modes shows that
the best-matching stellar models have ($M \approx 2.20, 2.35 M_{\odot}$) and
super-solar metallicity, in good agreement with previous measurements.
