  Radio magnetars are exotic sources noted for their diverse spectro-temporal
phenomenology and pulse profile variations over weeks to months. Unusual for
radio magnetars, the Galactic Center (GC) magnetar $\rm PSR~J1745-2900$ has
been continually active since its discovery in 2013. We monitored the GC
magnetar at $\rm 4-8~GHz$ for 6 hours in August$-$September 2019 using the
Robert C. Byrd Green Bank Telescope. During our observations, the GC magnetar
emitted a flat fluence spectrum over $\rm 5-8~GHz$ to within $2\sigma$
uncertainty. From our data, we estimate a $\rm 6.4~GHz$ period-averaged flux
density, $\overline{S}_{6.4} \approx (240 \pm 5)~\mu$Jy. Tracking the temporal
evolution of $\overline{S}_{6.4}$, we infer a gradual weakening of GC magnetar
activity during $2016-2019$ relative to that between $2013-2015.5$. Typical
single pulses detected in our study reveal marginally resolved sub-pulses with
opposing spectral indices, a feature characteristic of radio magnetars but
unseen in rotation-powered pulsars. However, unlike in fast radio bursts, these
sub-pulses exhibit no perceptible radio frequency drifts. Throughout our
observing span, $\rm \simeq 5~ms$ scattered pulses significantly jitter within
two stable emission components of widths, $\rm 220~ms$ and $\rm 140~ms$,
respectively, in the average pulse profile.
