GTK deprecations should continue to work on GTK3 which is all Geany and plugins support. So these warnings will likely not go away until a GTK4 port happens, so they should not be allowed fail a build.
For the Geanylua plugin, are you sure your build is using a version of the lua headers/library that is supported by the plugin? Note that geanylua is unmaintained and will not have been updated if type and function names in Lua headers have changed. Unless "somebody" with the time and ability takes the plugin under their wing it won't be updated to newer versions.
The project organiser incompatible pointer just needs an explicit cast @techee (As an aside, I thought incompatible pointers was in GCC 13, not 14)