Agreed that QT defaults looks sleek and that its probably not a wise choice for commercial projects unless you are already familiar with it and therefore can save a lot of coding time while reaping the multiplatform advantage.
But GTK+ can just as well be skinned with themes to look like anything you want it to, even on Windows.
Default theme since mid-2014 is named Adwaita and doesn't look that bad IMHO.
What you might be referring to is the pre-2014, downright ugly default named Raleigh which was created by Red Hat and carbon-14 dated back to the precambrian era...
All in all, UX Design is both an art and a science. Any toolkit can yield the best as well as the worst, given the right/wrong hands..!