If a reaction has negative $\Delta G$, can we then say with certainty that the reaction will be spontaneous?
Is negative $\Delta G$ the sole determinant for spontaneity?
For a spontaneous process at constant temperature and pressure, $\Delta G$ must (emphasis on must) be negative.
Spontaneity of a process , explicitly, depends on the enthalpy change and the entropy change. The Gibbs free energy change includes both these factors to predict the spontaneity of a process.
So, yes it can be said that Gibbs free energy change is the main factor that decides the spontaneity of a process.
Putting into better words, $\Delta G$ being negative is a consequence of a process being spontaneous.