The problem disappeared after I modified the KDE font settings.
My KDE font was Deja Vu Sans 8, when I changed it to Bitstream Vera Sans 8, GTK apps with Qtcurve had the correct font within KDE. Then I changed the font back to Deja Vu Sans 8 and still GTK apps with Qtcurve use the correct font.
The problem disappeared after I modified the KDE font settings.
My KDE font was Deja Vu Sans 8, when I changed it to Bitstream Vera Sans 8, GTK apps with Qtcurve had the correct font within KDE. Then I changed the font back to Deja Vu Sans 8 and still GTK apps with Qtcurve use the correct font.