Geany 1.37 switched to GTK3 by default, you can still build it with gtk2 by telling configure --enable-gtk2, but even that will go away in 1.38 (well thats the plan anyway) so references to GTK2 will slowly go away from Geany and plugins.
Note that technically GTK3 is ">=2.8.0" :grin:
See also reply on #1050