Unfortunately the first input dialog appearing in GFit seems to make GFit hang on more modern versions of Windows (beyond XP). A way around this is to assign the file type .dat (or .data, .tbl, .gfbin) to be opened by GFit by default. Then start GFit by double-clicking on e.g. the file demo.dat, enclosed in the installation of GFit. This starts GFit without an input dialog. Once GFit starts this way you may use menu File/Input New Data Set to get the input dialog and run GFit the normal way.