'gnatelim', described in manual,
http://www.pegasoft.ca/resources/boblap/6.html
is GNAT's only means of eliminating deadcode.

Let's try:

hello.adb:
-------------------------------

with ada.text_io;
use  ada.text_io;
procedure hello is
begin
    put("Hello World");
end hello;

-------------------------------

./gnatmake -c hello
./gnatbind hello
./gnatmake -f -c -gnatc -gnatt hello
./gnatelim hello.adb > gnat.adc

and guess what?

gnat.adc:
-------------------------------

---------------------------------------------------------
--  List of unused entities to be placed in gnat.adc.  --
---------------------------------------------------------
--  No unused entities.

-------------------------------

ELF executable is still ~200K stripped, just as before, no surprise.
Fucking scammers.