Temperature in the Gibbs Free Energy equation
In fact, Gibbs free energy is not defined at exact temperature. It is defined for standard states of all compounds in equation. So you really can use this solution of problem. However, it is a very simplified solution, because you assume that standard entropy and enthalpy do not change with temperature. This is not entirely correct assumption but if you took change of enthapy and entropy with temperature into account, it would be much more complicated. 
You can find information about Gibbs free energy here 
http://www.chem1.com/acad/webtext/thermeq/TE4.html#3A