Usually GUI builders produce bad source code, and even the produced GUI layout is not useable for most applications. Why? Most GUI builders only allow pixel exact layout, and not relative layout. In Tk terms, the layout code produced is using the "place" geometry manager, and not the "pack" or "grid" geometry managers. This works fine if all application users have the same font and screen settings, and if internationalization is not needed, but fails miserably if this is not the case.
Last time I looked at Prima the GUI builder was solving the problem by ignoring the user's font setting (as for example set by .Xdefaults on an X11 system), but forcing it's own fonts, resulting in ugly scaled fonts on my system.