  We report estimates of the cosmological parameters $\Omega_m$ and
$\Omega_{\Lambda}$ obtained using supernovae (SNe) associated with gamma-ray
bursts (GRBs) at redshifts up to 0.606. Eight high-fidelity GRB-SNe with
well-sampled light curves across the peak are used. We correct their peak
magnitudes for a luminosity-decline rate relation to turn them into accurate
standard candles with dispersion $\sigma = 0.18$ mag. We also estimate the
peculiar velocity of the low-redshift host galaxy of SN 1998bw, using
constrained cosmological simulations. In a flat universe, the resulting Hubble
diagram leads to best-fit cosmological parameters of $(\Omega_m,
\Omega_{\Lambda}) = (0.58^{+0.22}_{-0.25},0.42 ^{+0.25}_{-0.22})$. This
exploratory study suggests that GRB-SNe can potentially be used as
standardizable candles to high redshifts to measure distances in the universe
and constrain cosmological parameters.
