associate GS-Calc with xls

is it possible associate GS-Calc with xls files, in order to open them with double click.

Yes, it’s possible. You need to right-click some *.xls file, then in the “properties” dialog box choose “Opens with > Change” and specify the gscalc.exe file location.
You can also open all files (in the supported formats) by dragging them to the GS-Calc window.

i tried but i get this error:

You may need to move (for example, by using the simple Copy-Paste) the installation folder from the folder that only your Windows local account has access to, to some freely accessible location on a disk etc.
(Alternatively, you can just install GS-Calc as an admin which by default will suggest the common “Program Files” folder as the default “GS-Calc” installation folder.)