The development files are the include files needed to compile C programs, the PKG-CONFIG .pc files, the static libraries and a demo program (like gtk-demo)

.
|-- bin
|   `-- gdaui-demo-5.0
|-- include
|   `-- libgda-5.0
|       `-- libgdaui
|           [...]
`-- lib
|   |-- libgdaui-5.0.a
|   |-- libgdaui-5.0.la
|   `-- pkgconfig
|       `-- libgdaui-5.0.pc
`-- share
    `-- libgda-5.0
        `-- demo
            [...]