  Transmission spectra of exoplanets orbiting active stars suffer from
wavelength-dependent effects due to stellar photospheric heterogeneity.
WASP-19b, an ultra-hot Jupiter (T$_{eq}$ $\sim$ 2100 K), is one such strongly
irradiated gas-giant orbiting an active solar-type star. We present optical
(520-900 nm) transmission spectra of WASP-19b obtained across eight epochs
using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South
telescope. We apply our recently developed Gaussian Processes regression based
method to model the transit light curve systematics and extract the
transmission spectrum at each epoch. We find that WASP-19b's transmission
spectrum is affected by stellar variability at individual epochs. We report an
observed anticorrelation between the relative slopes and offsets of the spectra
across all epochs. This anticorrelation is consistent with the predictions from
the forward transmission models, which account for the effect of unocculted
stellar spots and faculae measured previously for WASP-19. We introduce a new
method to correct for this stellar variability effect at each epoch by using
the observed correlation between the transmission spectral slopes and offsets.
We compare our stellar variability corrected GMOS transmission spectrum with
previous contradicting MOS measurements for WASP-19b and attempt to reconcile
them. We also measure the amplitude and timescale of broadband stellar
variability of WASP-19 from TESS photometry, which we find to be consistent
with the effect observed in GMOS spectroscopy and ground-based broadband
photometric long-term monitoring. Our results ultimately caution against
combining multi-epoch optical transmission spectra of exoplanets orbiting
active stars before correcting each epoch for stellar variability.
