How ["03C0"] match <Ada Reference Manual>'s grammar in Annex P?
Maybe this question&answer can answer your question.
Which is your problem?
1. Can not type in the unicode character.
2. Can copy an unicode character but can not paste in GPS.
3. Can type or paste in GPS, but can not save.
4. Can type or paste in GPS and can save, but can not compile.
I guess your problem is the No. 4, then refer to my previous post as above link.
You need to use "gnatmake -gnatW8 Hello.adb" to compile.
Goto Build | Settings | Targets | Check Syntax
At the bottom insert -gnatW8 in the command field like:
%gnatmake -q -gnatW8 -c -gnats -u %eL -P%PP %X %fp
So does to Check Semantic if needed.