We could say that GTK has a rival: Qt.
It's interesting since LXQt was born from the LXDE maintainer being dissatisfied with GTK 3.[1] I've used both at some point - simple and lightweight.
What I've found is that Qt-based desktop environments (like LXQt, KDE) seem to do a good job at making sure GTK apps feel at home. Ubuntu MATE does a good job too by shipping the necessary configuration. Can't say the same for some 'bigger' desktops (-cough- Ubuntu, GNOME 3) where Qt apps use a generic 'default' theme and look out of place theme-wise.