Linux gui toolkits that is still "usable" today:
TK of Tcl
Swing of Java
EFL of Enlightment (the E desktop, full wayland support)
GNUStep from GNU guru used for objective-C (Wayland support in alpha version)
Motif → ancient Xorg stack
libXaw → X Athena Widgets
FLTK → Crossplatform, still use X
WxWidget is not a true toolkit, it is just a wrapper around other toolkits.