  Fundamental stellar parameters such as mass and radius are some of the most
important building blocks in astronomy, both when it comes to understanding the
star itself and when deriving the properties of any exoplanet(s) they may host.
Asteroseismology of solar-like oscillations allows us to determine these
parameters with high precision. We investigate the solar-like oscillations of
the red-giant-branch star $\gamma$ Cep A, which harbours a giant planet on a
wide orbit. We did this by utilising both ground-based radial velocities from
the SONG network and space-borne photometry from the NASA TESS mission. From
the radial velocities and photometric observations, we created a combined power
spectrum, which we used in an asteroseismic analysis to extract individual
frequencies. We clearly identify several radial and quadrupole modes as well as
multiple mixed, dipole modes. We used these frequencies along with
spectroscopic and astrometric constraints to model the star, and we find a mass
of $1.27^{+0.05}_{-0.07}$ M$_\odot$, a radius of $4.74^{+0.07}_{-0.08}$
R$_\odot$, and an age of $5.7^{+0.8}_{-0.9}$ Gyr. We then used the mass of
$\gamma$ Cep A and our SONG radial velocities to derive masses for $\gamma$ Cep
B and $\gamma$ Cep Ab of $0.328^{+0.009}_{-0.012}$ M$_\odot$ and
$6.6^{+2.3}_{-2.8}$ M$_{\rm Jup}$, respectively.
