Motif was already a bit of fragmentation. I remember working throug the Xt and OpenView manuals.
GTK started as a Motif clone, but then Lesstif came. There was XFORMS, never open, but fsf-b,essed.
What I do remember is that xforms gave me a pain in the shoulder because the scrollbars weren't quite rightly responsive.
These days, just use Qt, okay? It's the only sensible option.