Freebsd not supported ?

First test :

alr with gtkada
alr build

make: /SSD/home/x/.local/share/alire/builds/gtkada_25.0.1_d3787772/19bae168294ff0c68bce064d150edcb953a0d923dcece3cee2e5980785e88c1f/Makefile:112: warning: duplicate script for target "ifneq" ignored
make: Makefile:101: warning: using previous script for "ifneq" defined here
make: /SSD/home/x/.local/share/alire/builds/gtkada_25.0.1_d3787772/19bae168294ff0c68bce064d150edcb953a0d923dcece3cee2e5980785e88c1f/Makefile:113: Invalid line "endif"
make: Fatal errors encountered -- cannot continue
make: stopped making "src/gtkada-intl.adb" in /SSD/home/x/.local/share/alire/builds/gtkada_25.0.1_d3787772/19bae168294ff0c68bce064d150edcb953a0d923dcece3cee2e5980785e88c1f
warn: Execution failed for action: Post_Fetch run: make src/gtkada-intl.adb (from ${CRATE_ROOT}/.)
warn: Exit code: 1
warn: Action output not captured, check it above.
error: Compilation failed.

Second test :
cd /git/gtkada

./doinstall
Gtk+ binary package not found. Aborting the installation process.

But hey don't you do pkgconf ?

On my system :
pkgconf --cflags --libs gtk+-3.0

-I/usr/local/include/gtk-3.0 -I/usr/local/include/pango-1.0 -I/usr/local/include/cairo -I/usr/local/include/gdk-pixbuf-2.0 -I/usr/local/include/at-spi2-atk/2.0 -I/usr/local/include/at-spi-2.0 -I/usr/local/include/atk-1.0 -I/usr/local/libdata/pkgconfig/../../include/dbus-1.0 -I/usr/local/libdata/pkgconfig/../../lib/dbus-1.0/include -I/usr/local/include/libepoll-shim -I/usr/local/include/fribidi -D_THREAD_SAFE -I/usr/local/include/pixman-1 -I/usr/local/include/harfbuzz -I/usr/local/include/freetype2 -I/usr/local/include/libpng16 -I/usr/local/include/gio-unix-2.0 -pthread -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include -lgtk-3 -lgdk-3 -lz -lpangocairo-1.0 -lcairo-gobject -lgdk_pixbuf-2.0 -latk-1.0 -lpango-1.0 -lcairo -lharfbuzz -lgio-2.0 -lgobject-2.0 -lglib-2.0 -lintl

Reproduce the two reported paths: alr with gtkada followed by alr build, and ./doinstall from /git/gtkada. Inspect the generated Makefile around lines 101-113 and the installation check that reports no Gtk+ binary package, then compare the available pkgconf --cflags --libs gtk+-3.0 output. Done means the reported FreeBSD build and installation paths complete successfully.