Hello Sangrey,

I am not sure I understood correst your first questions; but I haven't tried to define G inf (if that's what you asked)

2. There is a problem in visualization if you are using other version of ANSYS rather than v19.0 (.1 or .2)
If the time is big in your case, right click on the x-axis of the graph and make it logarithmic scale - it is easier to judge the curve fitting.
One way to check curve fit is to build yourself in excel (or whatever) the curve using prony coefficient generated from ansys and compare them with G(t) of experimental results. The formulas are in ANSYS documentation at Visco-elastic section (I also put them in my previous comments)
Important note: G0 has a considerable effect in the results.