The GDX files are not text files, but binary files. The GAMS installation includes a GDX Viewer, which you can use exactly for the purpose you described, and you will also see there all the TIMES reporting parameters, input parameters, and even internal parameters and sets etc.
However, note that the "raw" equation marginals and shadow prices shown in the GDX file are discounted values, and, in the case of e.g. activities, flows, or period-wise equations, they correspod to the model periods. The TIMES reporting parameters provide you with the undiscounted and annualized values, which are much more useful and easier to process further.