Are all reactions with ΔG < 0 spontaneous?
No, Gibbs energy is only the relevant state function in the special case of constant temperature and pressure.  

In the wording of IUPAC: 


  partial derivative of Gibbs energy with respect to extent of reaction at constant pressure and temperature [is negative] for spontaneous reactions 


In the special case of constant temperature and volume, Helmholtz energy is the relevant state function.  

Most generally, a process is spontaneous if the entropy of the universe (system + surroundings) increases.