Ignoring enthalpy contribution in free Gibbs energy definition

The Gibbs free energy is defined as follows:

$Delta G = Delta H – TDelta S$

My professor and I are currently working with a simplified definition of $Delta G$ for reaction-diffusion systems in which we assume zero enthalpy contribution (for simplicity):

$Delta G = -TDelta S$

We are considering a fundamentally open thermodynamical system. In our work, this means that the system is open to the exchange of matter, but closed to the chemical species. Also, we assume constant temperature, volume, and pressure.

I am struggling with these assumptions because our model does not admit work or heat changes, although clearly, these quantities complement the definition of $Delta G$. So, my question is: how can we interpret the simplified definition of $Delta G$ physically? Is it correct to assume zero enthalpy contribution?

Edit:
Sorry I did not mention, but in fact we are using non-equilibrium thermodynamics formalism. The model we are using is the Brusselator. Hence, the system is closed to $X$ and $Y$ but open to $A$, $B$, $D$ and $E$.