Unfortunately I can’t seem to find anything for GtkRadiant 1.5, sorry.
If no one else can help you with this issue, then you can either reinstall GtkRadiant 1.5 or maybe also try the newest version GtkRadiant 1.6.4. Both available through here.
When I started mapping I remember using GTK 1.4 and for a short period of time 1.5, but I quickly switched over to NetRadiant. I’ve been using it for years now and it does feel way smoother to me. The biggest difference in handling is the frame-selection from NetRadiant compared to the click-hold-and drag selection in GtkRadiant, if that makes any sense. Maybe you could also try and give that one a go.
Cheers